Mitsubishi City Multi Communication: How M-NET Works
Before you troubleshoot a City Multi error code, understand the bus the system talks on. Mitsubishi City Multi systems, the PUHY and PURY series outdoor units on rooftops across every Australian capital, communicate over M-NET, the Mitsubishi Network protocol. Every outdoor unit, indoor unit, BC controller, MA remote controller and centralised controller shares this single two-wire transmission bus.
A few things about M-NET catch out techs who have only worked on Daikin. First, it is polarity-free: you can wire the two conductors either way and it will work. That does not make it immune to wiring problems. A damaged pair, a high-resistance joint, or a section run alongside a power cable will still cause faults. Second, City Multi VRF error codes are 4-digit (1302, 6607, 1102) and appear on the MA remote controller, the centralised controller (AE-200, GB-50, EW-50), or the seven-segment LED on the outdoor unit control board.
The one figure to keep in your head comes straight from the manual: on a No-response error the manufacturer tells you to check the transmission line for around 25V. Anything well outside that points at the bus rather than a single unit.
This guide covers the City Multi VRF 4-digit codes from the Mitsubishi service manuals. Every code below is drawn from the manufacturer's own cause and check lists. Residential 2-digit codes (E6, P4, U2 and the like on Mr Slim and splits) are a different system and are out of scope here.
Communication Faults
The codes that account for the majority of after-hours callouts on City Multi systems. M-NET is a two-wire transmission bus running through the building, and physical wires get damaged, corroded and pulled on by other trades. The manufacturer figure to remember: the transmission line should read around 25V.
6607
M-NET no acknowledgement (No ACK)
What it means
A device on the M-NET bus did not acknowledge receipt of a transmission within the expected window. The address shown alongside 6607 identifies the unit that failed to respond, which is where to start looking.
What to check first
Faulty or disconnected transmission wiring: Loose terminal, broken conductor, or unseated connector at the unit identified by the address. The specific connector to check depends on source type: CN02 on BC controllers, CN2M on indoor units, CN1 on LOSSNAY. Most callouts on 6607 land here. Incidental cause that clears on full system power cycle: Brief bus glitch or microcontroller wedge at the source. Power down outdoor, indoor, BC, and LOSSNAY units together for at least 5 minutes, then bring them back up. If 6607 returns, work through the remaining causes in order. Address changed at the unresponsive unit during operation: An indoor unit, ME remote, BC controller, or LOSSNAY had its M-NET address modified while the system was running. Peers still hold the old address in memory and ACKs against the new address fail. Confirm address consistency across the bus then power-cycle to refresh peer memory. Defective controller at the unresponsive unit: The board at the address alongside 6607 has failed. Replace only after wiring, length, address, and CN40 checks come back clean. If 6607 shows for an indoor unit but only some indoor units are affected, the failed board is on one of the affected units. Transmission wiring exceeds physical specification: Total bus length over 200m, remote-controller branch over 10m (12m on some controllers), or wire diameter under 1.25mm² (AWG16). Voltage drop and signal degradation past these limits make ACK unreliable. Verify against the as-built wiring layout. CN40 misconfiguration on multi-outdoor systems: Outdoor unit's male power-supply connector not seated on CN40, OR multiple outdoor units have male connectors on CN40 simultaneously. Centralised-control bus voltage drops below 20V and ACKs fail. Suspect this when 6607 shows in its 'all error source addresses' form. Once corrected, expect follow-on errors 7100, 7101, 7102, or 7105 to surface as the system re-syncs.
Nexus iQ advantage: M-NET drops are intermittent and almost impossible to catch on a single site visit. The platform logs every No-ACK event with a timestamp and the source address, so you arrive knowing which unit stopped responding and whether it correlates with an external trigger.
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6608
M-NET no response error
What it means
A device on the M-NET bus acknowledged the transmission but did not return the expected response data. Most callouts on this code trace back to noise on the transmission line.
What to check first
Noise on the transmission line: Electrical interference (variable-speed drives, switching gear, parallel power runs, faulty fluorescent ballasts) corrupting the response data after a clean ACK. Check the transmission waveform per the manual's noise diagnostic procedure. The manual flags this as the most likely cause of 6608. Transmission wiring exceeds physical specification: Total bus length over 200m, remote-controller branch over 12m, or wire diameter under 1.25mm² (AWG16). Voltage drop and signal degradation past these limits let ACK through but corrupt the longer response packets that follow. Verify against the as-built wiring layout. Transmission line work performed while powered: Bus modifications made without first powering down indoor and outdoor units. Transmitted data collides with the work in progress and the response waveform corrupts. Power down all units together for 5 minutes, complete the work, then bring them back up.
Nexus iQ advantage: Because noise is the most likely cause of 6608, continuous logging of the transmission line lets you correlate dropouts with the plant that switches on at the same moment, rather than chasing an intermittent fault blind.
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6600
Duplicate M-NET address
What it means
Two or more devices on the M-NET bus are configured with the same address. The unit detecting the conflict cannot reliably exchange commands with its peers until the duplicate is corrected.
What to check first
Two or more units share an M-NET address: Outdoor units, BC controllers, indoor units, LOSSNAY units, or ME remote controllers configured with the same address. Find the duplicate, correct one of the addresses, then power down all units for at least five minutes before turning them back on. Noise distorting transmission signals: Electrical interference on the M-NET line corrupting valid signals so they appear to come from a duplicate address. Suspect this when the system continues to operate normally despite 6600 latching. Check the transmission waveform and noise per the manual's transmission diagnostic procedure.
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6602
M-NET transmission processor hardware error
What it means
A transmission processor sent a '0' on the M-NET line but read back a '1'. The bit on the wire does not match what the processor transmitted, indicating the line is being driven by something else.
What to check first
Transmission wiring or polarity changed while powered: Bus work performed without first powering down indoor and outdoor units. Transmitted data collides with the work in progress, the waveform corrupts, and the error latches. Power down all units, then bring them back up. Power supply connector misconfiguration at CN40: On multi-outdoor-unit systems, the male power-supply connector is plugged into the female CN40 switch on more than one outdoor unit. Or, on systems with a separate transmission-line power supply, the male connector is on CN40 instead of CN41. Only one path should energise the bus. Earth fault on the transmission line: Shielded conductor contacting one of the signal conductors, or a signal conductor shorted to earth. Inspect terminations and recent cable runs. Defective controller at the error source: The board identified by the address alongside 6602 has failed. Replace only after wiring, CN40 connections, and noise have been ruled out. Noise on the transmission line: External electrical interference corrupting the signal. Check the transmission waveform per the manual's noise diagnostic procedure if the wiring and CN40 checks come back clean.
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6603
M-NET transmission bus busy
What it means
A transmission processor has been unable to send a command for between 4 and 10 minutes continuously because the M-NET line was busy. Most callouts on this code are noise on the line rather than legitimate traffic.
What to check first
Continuous noise on the transmission line: Short-wavelength interference (variable-speed drives, switching gear, parallel power runs, faulty fluorescent ballasts) keeping the line apparently busy. Check the transmission waveform per the manual's noise diagnostic procedure to identify the source. Defective controller at the error source: If the line is clean and no noise is found, the controller identified by the address alongside 6603 has failed and is corrupting the bus. Replace only after noise checks come back clean.
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6831
MA remote controller signal reception error (no signal)
What it means
Communication between the MA remote controller and the indoor unit failed; no proper data was received for 3 minutes.
What to check first
Contact failure on the MA remote controller lines: Check for disconnected or loose transmission lines at the indoor units or MA remote controllers. Main/sub setting error: One MA remote controller must be set to MAIN and the rest to SUB; correct any main/sub or duplicate-address conflict. Wiring regulations not met: Confirm the MA remote controller capacity limit, wire length, wire size and number of controllers and indoor units. Noise interference: Check the transmission waveform and the MA remote controller line for electrical noise (see manual section 8-4). Indoor board or MA remote controller circuit fault: If no problems are found, replace the indoor unit board or the MA remote controller. LED1 lit = indoor main power on; LED2 lit = MA controller line powered.
Nexus iQ advantage: MA remote controller dropouts are logged with a timestamp, so an intermittent controller-line fault shows its pattern instead of clearing before you arrive.
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The same fault, on your phone
Know about 6607 before the tenant does.
A Nexus controller sits on the M-NET and logs every fault the moment it happens: the code, the unit address, and the time. It lands in the AirNexus app before anyone calls you.
Supplied by AirNexus to HVAC partners across Australia.
Pressure and Temperature Protection
These trips stop the compressor to protect it. The trap is telling a genuine out-of-range condition apart from a sensor that has failed, so confirm the reading before condemning hardware.
1302
High pressure fault
What it means
During operation the outdoor unit stops when the pressure sensor reads 3.78MPa [548psi] or higher; a third detection of 3.87MPa [561psi] within 30 minutes causes an error stop with 1302. The unit also error-stops immediately if the pressure switch detects 4.15 MPa [601psi]. At startup, a reading of 0.098MPa [14psi] or lower also triggers 1302.
What to check first
Indoor unit or BC controller LEV / solenoid valve malfunction: Perform a heating operation and check the LEVs and solenoid valves (indoor LEV, BC LEV3, SVM, SVA/C, SVB, SV4a-d). See manual section 8-8. Port address setting error: Confirm the port address of the indoor unit. Refrigerant service valve actuation failure: Confirm that the refrigerant service valve is fully open. Reduced indoor condensing capacity (short cycle, clogged filter, dirty fan or heat exchanger, indoor fan failure): Check the indoor units for problems and correct them. Reduced outdoor condensing capacity (short cycle, dirty heat exchanger, outdoor fan failure): Check the outdoor units and the outdoor fan (see manual section 8-7). Solenoid valve (SV1a) malfunction: The bypass valve SV1a cannot control the rise in high pressure (see manual section 8-6). The manual lists 2 further causes.
Nexus iQ advantage: Discharge pressure and condenser approach temperature are trended daily. A gradual climb over weeks tells you the coil is fouling, so a clean can be scheduled before 1302 trips on the hottest day of the year.
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1301
Low pressure fault
What it means
When the compressor starts from Stop Mode for the first time, if low pressure reads 0.098MPa [14psi] immediately before start-up, the operation immediately stops.
What to check first
Inner pressure drop due to a leak: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3). Low pressure sensor failure: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3). Short-circuited pressure sensor cable due to torn outer rubber: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3). Missing pin on the male connector: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3). Disconnected wire: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3). Low-pressure input circuit failure on the controller board: Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3).
Nexus iQ advantage: Suction pressure and subcooling are trended continuously, so a slow charge loss is visible on the diagnostics chart weeks before 1301 trips at start-up.
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1102
Discharge temperature fault
What it means
The outdoor unit stops when the discharge temperature reaches 120C [248F] or more. Repeated detection within 30 minutes leads to an error stop with 1102; preliminary errors (1202) show on the LED for 30 minutes after a stop.
What to check first
Gas leak or gas shortage: Evaluate and adjust the refrigerant charge (see manual section 6-9). Overload operation: Check operating conditions and the operation status of the indoor and outdoor units. Indoor unit or BC controller LEV / solenoid valve malfunction: Perform a heating operation and check the LEVs (indoor LEV, BC controller LEV1/LEV3, SVM1/2, SVA/C, SVB, SV4a-d). See manual section 8-8. Port address setting error: Confirm the port address of the indoor unit. Closed ball valve: Confirm that the ball valve is fully open. Outdoor fan or fan controller failure: Check the outdoor unit fan (see manual section 8-7). The manual lists 2 further causes.
Nexus iQ advantage: Discharge temperature is one of the most valuable data points on a VRF system. The platform tracks it against compressor frequency and ambient temperature and shows it climbing in the Diagnostics view, long before the 1102 lockout.
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1500
Refrigerant overcharge
What it means
Detected by discharge temperature superheat. If TdSH <= 10C [18F] is satisfied during operation and again within 30 minutes, the unit makes an abnormal stop with 1500; the preliminary code is 1600.
What to check first
Overcharged refrigerant: Evaluate and adjust the refrigerant charge (see manual section 6-9). Thermistor input circuit failure on the control board: Check the temperature and pressure on the LED monitor. Faulty mounting of thermistor (TH4): Check the TH4 reading on the LED monitor.
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Inverter and Compressor Faults
These relate to the compressor and inverter, the most expensive components in the outdoor unit. Not every one means a compressor replacement: several come back to supply, airflow or a sensor. Discharge the DC bus and verify with a meter before any connector work on these boards.
4250
IPM error / overcurrent
What it means
The inverter detected an overcurrent, an IPM/IGBT short or ground fault, a short-circuited motor, or a DCL overcurrent, via the current sensor or the IPM error signal. The preliminary code is 4350.
What to check first
Inverter output related: Check the inverter board error detection circuit, the compressor for ground fault and coil resistance, the inverter for damage, the site conditions and the IGBT module (see manual section 8-10). Grounding fault or short-circuited compressor: Check the compressor for a ground fault and coil resistance (see manual section 8-10-3). Output wiring short circuit: Check the output wiring for a short circuit. Model selection switch (SW5-3 to SW5-8) set incorrectly: Check the model selection dip switches on the outdoor unit control board.
Nexus iQ advantage: Inverter current is trended against compressor frequency. A rising current-to-frequency ratio points to increasing mechanical resistance, visible months before a 4250 overcurrent lockout.
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4220
Abnormal bus voltage
What it means
An abnormal inverter DC bus voltage: a drop below 289V (detail 108/131), a rise to 830V or above (109), a hardware over-voltage (110), a logic error (111/112), a voltage-boost control error (123), or a bus-circuit relay fault (124). The preliminary code is 4320.
What to check first
Power supply environment: Check for a momentary power failure and that the phase-to-phase voltage (L1-L2, L2-L3, L1-L3) is above 342V. Inverter main-circuit fault: With the inverter stopped, check the FT-P/FT-N (or SC-P/SC-N) voltage against 420V and inspect the noise filter coil, the noise-filter-to-INV and INV-to-DCL wiring, the inrush resistor and 72C; replace the INV board if no problem is found. See manual section 8-10. Different voltage connection (bus voltage rise): Check the power supply voltage at TB1. External noise (logic error): Check the inverter board error detection circuit (see manual section 8-10-2). Control board failure: Check that 12VDC reaches CN72 on the control board during inverter operation; check fuse F01 and replace the control board if needed.
Nexus iQ advantage: DC bus voltage is monitored continuously, so an intermittent supply sag or a developing capacitor problem is flagged before it becomes a hard 4220 stop.
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4230
Heatsink overheat protection
What it means
The heatsink temperature (THHS) reached its protection threshold (INV20Y 100C, INV30YC 94C), or on INV30YC the DCL temperature reached 170C (detail 126). The preliminary code is 4330.
What to check first
Fan board failure: Check the fan board error detection circuit and inverter (see manual sections 8-10-8 to 8-10-10). Outdoor unit fan failure: Check the outdoor fan operation and the fan motor (see manual section 8-10-7). Air passage blockage: Check that the heat sink cooling air passage is not blocked. THHS sensor failure: Check the IGBT mounting and the THHS reading on the LED monitor; replace the INV board if abnormal. DCL temperature sensor or DCL fault (INV30YC): Check connector CNTH on the inverter board and replace the DCL if it persists.
Nexus iQ advantage: Heatsink temperature is trended against compressor frequency. If it starts climbing at a frequency where it was previously stable, it points to a blocked air passage or a failing fan before the 4230 trip.
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4240
Overload protection
What it means
The output current (Iac) exceeded Imax, or THHS exceeded TOL, continuously for 10 minutes during inverter operation. The preliminary code is 4340.
What to check first
Air passage blockage: Check that the heat sink cooling air passage is not blocked. Power supply environment: Confirm the power supply voltage is 342V or above. Inverter failure: Troubleshoot the inverter (see manual section 8-10). Compressor failure: Check that the compressor has not overheated and check the oil return section (see manual section 8-10-3). Model selection switch (SW5-3 to SW5-8) set incorrectly: Check the model selection dip switches on the outdoor unit control board.
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5301
ACCT sensor / inverter current fault
What it means
The inverter ACCT current-sensor circuit detected a fault: output current below 1.5Arms for 10 seconds (115), an ACCT circuit error (117), insufficient current at self-diagnosis (119), an improperly mounted ACCT (120), or a DCL current-circuit error (127). The preliminary code is 4300.
What to check first
Inverter open output phase or output wiring problem: Check the output wiring; confirm the U- and W-phase cables pass through CT12 and CT22 on the INV board. Compressor failure: Check the compressor for a ground fault and coil resistance (see manual section 8-10-3). INV board failure: Check the inverter board error detection circuit and the inverter for damage (see manual sections 8-10-2, 8-10-4, 8-10-5). DCL current circuit contact or wiring fault: Check the wiring between CNCT4A and CNCT4B and the SC-L terminal; replace the INV board if it persists.
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4102
Open phase
What it means
An open phase of the power supply (L1 or N) detected at power-on, or an L2/N open phase at the start of operation; the L3 phase current is outside the specified range. The preliminary code is 4152.
What to check first
Power supply problem (open phase or voltage drop): Check the input voltage at the power supply terminal block TB1. Noise filter coil or board problem: Check the coil connections and for burnout; confirm CN3 is 198V or above and noise-filter CN4 pins 3-5 are 198V or above. Wiring failure: Confirm the control board CNAC is 198V or above; check the noise-filter-to-control-board wiring and that noise filter TB23 to INV board SC-L3 passes through CT3. Blown fuse: Check fuse F01 on the control board and F1/F2 on the noise filter; if blown, check for a short circuit or earth fault of the actuator. CT3 failure: Replace the inverter board if this is detected after the compressor has started. Control board failure: Replace the control board if none of the above is the cause.
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Sensor Faults
City Multi systems use a lot of thermistors and pressure transducers, and like every electronic component they fail. Sensor faults are usually a quick diagnosis: measure resistance, check the connector, replace the part. The trap is a reading that looks wrong when the sensor is fine and the real problem is elsewhere.
5101
Return air / OA-inlet thermistor fault (TH21 / TH4)
What it means
A short (90C [194F] or above) or open (-40C [-40F] or below) of the return air thermistor (indoor TH21) or the OA processing unit inlet thermistor (TH4) detected during thermostat ON. The preliminary code is 1202.
What to check first
Thermistor failure: Check the thermistor resistor: 0C 15 kOhm, 10C 9.7 kOhm, 20C 6.4 kOhm, 30C 4.3 kOhm, 40C 3.1 kOhm. Connector contact failure: Check the connector contact. Disconnected or partially disconnected thermistor wire: Check the thermistor wire. Unattached thermistor or contact failure: Check that the thermistor is mounted and making contact. Indoor board (detection circuit) failure: When no fault is found, the indoor board is a failure.
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5104
Intake / outside / discharge thermistor fault (TH1 / TH24 / TH4)
What it means
A short or open of the OA intake thermistor (TH1), the all-fresh indoor outside thermistor (TH24), or the outdoor discharge thermistor (TH4). The preliminary code is 1202.
What to check first
Thermistor failure: Check the thermistor resistance. Pinched lead wire: Check for a pinched lead wire. Torn wire coating: Check the wire coating. Missing pin on the male connector or contact failure: Check the connector. Disconnected wire: Check for a disconnected wire. Thermistor input circuit failure on the control board: Check the sensor temperature on the LED monitor. When it is far different from the actual temperature, replace the control board.
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5107
Outside temperature thermistor fault (TH7)
What it means
A short or open of the outdoor outside-temperature thermistor (TH7). The preliminary code is 1221.
What to check first
Thermistor failure: Check the thermistor resistance. Pinched lead wire: Check for a pinched lead wire. Torn wire coating: Check the wire coating. Missing pin on the male connector or contact failure: Check the connector. Disconnected wire: Check for a disconnected wire. Thermistor input circuit failure on the control board: Check the sensor temperature on the LED monitor. When it is far different from the actual temperature, replace the control board.
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5110
Heatsink thermistor fault (THHS)
What it means
A short or open of the heatsink thermistor (THHS) detected just before or during inverter operation. The preliminary code is 1214.
What to check first
INV board failure: If the fault recurs when the unit is operated, replace the INV board.
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5201
High pressure sensor fault (63HS1 / BC PS1)
What it means
The high pressure sensor (outdoor 63HS1) reads 0.098MPa [14psi] or less during operation, or the BC controller PS1 reads 4.06MPa [589psi] or above or 0.98MPa [142psi] or below.
What to check first
High pressure sensor failure: Compare the high-pressure sensor reading with gauge pressure (see manual section 8-5-1). Pressure drop due to a refrigerant leak: Compare the high-pressure sensor reading with gauge pressure (see manual section 8-5-1). Torn wire coating: Check for damaged wire coating. Missing pin on the male connector or contact failure: Check the connector pin. Disconnected wire: Check for a disconnected or broken wire. High-pressure input circuit failure on the control board: Check the sensor reading on the LED monitor; replace the control board if it is far from actual.
Nexus iQ advantage: High and low pressure sensor readings are compared against the values expected for current operating conditions, flagging sensor drift before it causes a nuisance trip.
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Drain, Setting and Address Faults
Less common on the callout sheet but worth recognising on sight. A2 drain faults are frequent in humid coastal conditions; setting and address faults usually surface at or just after a system change.
2502
Drain pump fault
What it means
On models with a drain sensor, self-heating of the drain sensor interpreted as immersion, or on float-switch models the float held ON for 3 minutes, is a drain pump error and 2502 appears.
What to check first
Drain pump failure: Check the drain pump for proper functioning. Drain water drainage problem: Check for proper drainage (clogged drain pump or piping). Water adhering to the drain sensor, or a stuck or failed float switch: Check the lead-wire routing and for a clogged filter; check the float switch operation and its resistance with it on and off. Indoor unit control board failure: Replace the indoor unit control board if the drain pump drive or float switch input circuit is faulty. Indoor unit electronic valve leak occurring simultaneously: Check the solenoid valves on the indoor unit for leaks.
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7100
Indoor capacity total exceeds outdoor limit
What it means
The combined capacity of all indoor units connected to one outdoor unit exceeds the maximum the outdoor model is rated for. The system cannot start until the configuration is corrected. Often surfaces after a 6607 / CN40 fix on multi-outdoor systems, as the outdoor unit re-syncs and recounts the connected indoor capacity.
What to check first
Indoor capacity total exceeds outdoor model limit: Sum of Qj capacity codes from connected indoor units (Qj 3 for size 15 up to Qj 50 for size 250) exceeds the outdoor model's limit. Either too many indoor units have been added, or one or more indoor capacity codes (SW2 on indoor board) is set higher than the actual unit. Outdoor model selection switches set incorrectly: Dipswitches SW5-3 through SW5-8 on the outdoor control board do not match the actual outdoor model. Verify against the model-to-switch table in the manual; common after a control board swap or reset. OC and OS not properly connected at TB3: On systems with an outdoor sub-unit (OS), the TB3 terminal block linking the outdoor controller (OC) and OS is loose, miswired, or has a broken conductor. Confirm both ends are tight and continuity is good.
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7101
M-NET capacity code mismatch
What it means
An indoor or outdoor unit is reporting a capacity code that does not match what the system expects. The M-NET system rejects the configuration so the affected unit will not start. Often surfaces after a 6607 / CN40 fix on multi-outdoor systems, as capacity codes are revalidated.
What to check first
Indoor capacity code on SW2 set wrong: SW2 on the indoor controller board is set to a capacity code that does not match the actual indoor model. Verify against the indoor model-to-Qj table; correct, then power-cycle outdoor and indoor units together. Outdoor model selection switches set incorrectly: Dipswitches SW5-3 through SW5-8 on the outdoor control board do not match the actual outdoor model. Confirm against the model-to-switch table; common after a control board swap or reset.
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7130
Indoor unit refrigerant family mismatch
What it means
An indoor unit configured for an older refrigerant family (R22 or R407C) is connected to a system that the BC controller is set up to handle differently. The two are incompatible and the system cannot start.
What to check first
Connected indoor unit is for an incompatible refrigerant: An R22 or R407C indoor unit has been connected to a system designed for a different refrigerant family. Confirm the indoor model on the spec plate against the outdoor and BC controller refrigerant. If the indoor is genuinely incompatible, it cannot remain on this system. BC controller misconfigured at TYP1 / DIP switches: TYP1 connector on the BC control board is unseated or wired incorrectly, or DIP switches SW4-1, SW5-7, or SW5-8 on the BC control board are set wrong. Causes the BC to recognise the wrong indoor refrigerant family. Inspect connector and verify switch settings against the manual. M-NET connection adapter wired to indoor unit instead of outdoor: On systems that include Slim Model (A control) indoor units, the M-NET connection adapter must be wired to the outdoor unit, not the indoor. If wired to the indoor side, the BC controller misidentifies the refrigerant family and 7130 latches.
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Tired of chasing City Multi codes?
Nexus iQ monitors discharge temperature, pressures, compressor frequency and M-NET health 24/7. It catches the patterns that lead to fault codes before they trip.
See How It WorksWhat Monitoring Catches That Fault Codes Miss
City Multi error codes are reactive. By the time a code appears, the problem has already happened. Every system generates dozens of parameters that change continuously: discharge temperature, high and low pressure, compressor frequency, LEV position, M-NET health. In a healthy system these hold consistent relationships. When something starts to degrade, a slow leak, a fouling coil, a controller line picking up noise, the relationships shift long before any single parameter crosses a protection threshold.
Nexus iQ captures this data continuously and compares each system against its own history, not a generic model. When the patterns diverge you get a diagnostic insight, not a lockout.
| Scenario | Without Monitoring | With Nexus iQ |
|---|---|---|
| Slow refrigerant leak | 1301 low pressure on the coldest morning. Emergency callout. | Low-pressure trend flagged weeks earlier. Leak found during scheduled service. |
| Condenser fouling | 1302 high pressure lockout on the hottest day. System down for hours. | Discharge pressure trending up. Coil clean scheduled before summer peak. |
| Compressor stress building | 4250 overcurrent. Compressor replacement risk. | Current-to-frequency trend flagged months earlier. Planned repair. |
| M-NET intermittent | Random 6607 faults. Multiple callouts, no pattern found. | No-ACK events logged with the source address. Pattern identified on the first visit. |
| Discharge temperature climbing | 1102 discharge temperature lockout. | TH4 trend against frequency flagged. Charge or LEV checked before the stop. |
The most expensive City Multi fault code is the one that trips on a Friday afternoon in January. Monitoring does not prevent failures, it prevents surprises.
Quick Reference: Every City Multi VRF Code
Use this searchable table to find any City Multi 4-digit code. Type a code (e.g. 1302), a keyword (e.g. pressure), or a category. Click any code to jump to its detail where one is provided above.
| Code | Category | Description | Severity |
|---|---|---|---|
| 0403 | Communication | Serial communication error / panel communication error | Medium |
| 0404 | Communication | Indoor unit EEPROM abnormality | Medium |
| 1102 | Pressure | Discharge temperature fault | High |
| 1301 | Pressure | Low pressure fault | High |
| 1302 | Pressure | High pressure fault | High |
| 1500 | Pressure | Refrigerant overcharge | High |
| 2500 | Drain | Drain sensor submergence | Medium |
| 2502 | Drain | Drain pump fault | Medium |
| 2503 | Drain | Drain sensor (Thd) fault | Medium |
| 2600 | Drain | Water leakage (humidifier) | Low |
| 2601 | Drain | Water supply cutoff (humidifier) | Low |
| 3121 | Sensor | Out-of-range outside air temperature | Medium |
| 4102 | Power | Open phase | High |
| 4106 | Power | Transmission power supply fault | High |
| 4109 | Power | Indoor unit fan operation error | Medium |
| 4115 | Power | Power supply signal sync error | High |
| 4116 | Power | RPM error / motor error | Medium |
| 4121 | Power | Function setting error (outdoor) | Medium |
| 4124 | Power | Electric system stopped by damper abnormality | Medium |
| 4220 | Inverter | Abnormal bus voltage | High |
| 4225 | Inverter | Abnormal bus voltage (fan inverter) | High |
| 4226 | Inverter | Abnormal bus voltage (fan box) | High |
| 4230 | Inverter | Heatsink overheat protection | High |
| 4240 | Inverter | Overload protection | High |
| 4250 | Inverter | IPM error / overcurrent | Critical |
| 4255 | Inverter | IPM error / overcurrent (fan inverter) | Critical |
| 4256 | Inverter | IPM error / overcurrent (fan box) | Critical |
| 4260 | Inverter | Heatsink overheat protection at startup | High |
| 5101 | Sensor | Return air / OA-inlet thermistor fault (TH21 / TH4) | Medium |
| 5102 | Sensor | Pipe thermistor fault (TH22 / TH2) | Medium |
| 5103 | Sensor | Gas-side pipe / heat-exchanger-outlet thermistor fault (TH23 / TH3) | Medium |
| 5104 | Sensor | Intake / outside / discharge thermistor fault (TH1 / TH24 / TH4) | Medium |
| 5105 | Sensor | Accumulator inlet thermistor fault (TH5) | Medium |
| 5106 | Sensor | Heat exchanger inlet thermistor fault (TH6) | Medium |
| 5107 | Sensor | Outside temperature thermistor fault (TH7) | Medium |
| 5109 | Sensor | Continuous heating thermistor fault (TH9) | Medium |
| 5110 | Sensor | Heatsink thermistor fault (THHS) | Medium |
| 5111 | Sensor | Continuous heating / BC liquid inlet thermistor fault (TH11) | Medium |
| 5112 | Sensor | Continuous heating / BC bypass outlet thermistor fault (TH12) | Medium |
| 5115 | Sensor | BC controller LEV3 outlet thermistor fault (TH15) | Medium |
| 5116 | Sensor | BC controller LEV3 inlet thermistor fault (TH16) | Medium |
| 5120 | Sensor | DCL temperature sensor fault (THL) | Medium |
| 5201 | Sensor | High pressure sensor fault (63HS1 / BC PS1) | Medium |
| 5203 | Sensor | BC controller intermediate pressure sensor fault (PS3) | Medium |
| 5301 | Inverter | ACCT sensor / inverter current fault | Critical |
| 5305 | Inverter | Fan motor position / RPM error | High |
| 5306 | Inverter | Fan motor position / RPM error (fan inverter 2) | High |
| 5701 | Drain | Loose float switch connector | Medium |
| 6201 | Controller | Remote controller board fault (nonvolatile memory) | Low |
| 6202 | Controller | Remote controller board fault (clock IC) | Low |
| 6600 | Communication | Duplicate M-NET address | Medium |
| 6601 | Communication | M-NET polarity setting error | Medium |
| 6602 | Communication | M-NET transmission processor hardware error | Medium |
| 6603 | Communication | M-NET transmission bus busy | Medium |
| 6606 | Communication | M-NET internal processor communication error | Medium |
| 6607 | Communication | M-NET no acknowledgement (No ACK) | Medium |
| 6608 | Communication | M-NET no response error | Medium |
| 6831 | Communication | MA remote controller signal reception error (no signal) | Medium |
| 6832 | Communication | MA remote controller signal transmission error (synchronization) | Medium |
| 6833 | Communication | MA remote controller signal transmission error (hardware) | Medium |
| 6834 | Communication | MA remote controller signal reception error (start bit) | Medium |
| 6840 | Communication | A-control communication reception error | Medium |
| 6841 | Communication | A-control communication synchronism not recovered | Medium |
| 6842 | Communication | A-control transmit/receive hardware trouble | Medium |
| 6843 | Communication | A-control communication start bit detection error | Medium |
| 6846 | Communication | A-control start-up time over | Medium |
| 7100 | Setting | Indoor capacity total exceeds outdoor limit | Medium |
| 7101 | Setting | M-NET capacity code mismatch | Medium |
| 7102 | Setting | Indoor unit count wrong or transmission down | Medium |
| 7105 | Setting | M-NET outdoor or BC address out of range | Medium |
| 7106 | Setting | Attribute setting error | Medium |
| 7107 | Setting | Port setting error | Medium |
| 7110 | Setting | Outdoor-to-indoor connection signal lost | Medium |
| 7111 | Setting | Wireless remote sensor not available | Medium |
| 7113 | Setting | CNTYP function-setting error (outdoor or BC) | Medium |
| 7117 | Setting | Outdoor model identification error | Medium |
| 7130 | Setting | Indoor unit refrigerant family mismatch | Medium |
Getting Started
Knowing what a City Multi code means is step one. Preventing it is step two. Here is how to move from reactive to predictive maintenance on your City Multi systems.
1. Book a Demo
See Nexus iQ monitoring a live City Multi system: real fault history, real diagnostic charts, real trending data on a real building. No slides, no hypotheticals.
2. Connect Your System
A Nexus controller connects directly to the Mitsubishi M-NET bus. It reads every data point the system generates, temperatures, pressures, frequencies, LEV positions, error codes and runtime hours, and streams it to the Nexus iQ cloud platform in real time.
3. Stop Guessing
Within 24 hours you have a complete diagnostic view of every outdoor and indoor unit. Within two weeks the platform has enough baseline data to flag anomalies. Within a month you receive predictive insights, the subtle pattern shifts no fault code would ever catch.
Sources
The code meanings and diagnostic steps in this guide are drawn from the following Mitsubishi Electric City Multi service documents.
- Mitsubishi Electric City Multi VRF Service Manual HWE1610A
- Mitsubishi Electric City Multi Service Handbook HWE1314A (PURY-P-Y(S)LM-A1)
- Mitsubishi Electric City Multi Fault Code List, dated 20241024
Mitsubishi City Multi fault codes: frequently asked questions
What does Mitsubishi error code 1302 mean?
1302 is a high pressure fault on the outdoor unit. The pressure sensor read 3.78 MPa (548 psi) or higher, or the pressure switch tripped at 4.15 MPa (601 psi). Check the indoor and BC controller LEVs and solenoid valves, confirm the refrigerant service valve is fully open, and check the outdoor coil, fan and charge.
What does Mitsubishi error code 6607 mean?
6607 is a No ACK error: a device on the M-NET bus did not acknowledge a transmission. The address shown with 6607 identifies the unit that did not respond. Check that unit's transmission wiring and connector, confirm the transmission line reads around 25V, and power the system down for at least 5 minutes before replacing any board.
Can I monitor Mitsubishi City Multi fault codes remotely?
Yes. The AirNexus Nexus iQ platform connects to Mitsubishi City Multi systems and shows the fault in Findings & Insights and the Diagnostics view when the unit reports it, so you can see what is wrong before anyone goes to site. The AirNexus app also sends a push notification when a fault happens. Search the fault code lookup.