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TECHNICAL 13 min read

Trane VRF Fault Codes (United States): The Field Guide

Every Trane US VRF error code from the Mitsubishi City Multi self-diagnosis set. What it means, what to check first. Built for US techs who need answers on site.

By AirNexus™ · Intelligent HVAC Platform

Trane VRF Systems (United States): How the Codes Read

These codes apply to Trane VRF sold in the United States (208-230 V and 460 V, 60 Hz), which is Mitsubishi Electric City Multi supplied through METUS (Mitsubishi Electric Trane HVAC US); Trane VRF sold in Australia and Europe is a different platform with a different code set, so do not use this page for those units. A fault shows as a 4-digit code on the outdoor unit seven-segment display, the PAR-series MA remote, or the AE-200 / AE-50 / EW-50 centralised controller.

The first digit points you at the system: 1 is the refrigerant cycle (pressure and temperature), 2 is water and drain, 4 is the inverter and power, 5 is a sensor, 6 is M-NET communication, and 7 is system and configuration. A common trap: the 6xxx codes are communication, not a pressure fault, and 6607 usually names the address of the unit that stopped answering.

Which Trane is this? US City Multi outdoor units are the PURY (R2 heat recovery), PUHY (Y heat pump) and PQHY / PQRY (water-cooled) families, with indoor units such as PKFY, PLFY, PMFY, PCFY, PEFY, PFFY and PVFY, and controllers AE-200A / AE-50A, EW-50A and the PAR-series MA remote. The earlier Trane VRF (4TVH / 4TVR, 2013, VRF-SVN34A-EN) was a different Samsung-based platform and is not covered here.

Communication Faults

The codes you will see most often on City Multi callouts. Trane US VRF runs the M-NET communication bus between outdoor units, BC controllers, indoor units and controllers, and most of these come back to wiring, polarity, a terminating resistor or addressing. Codes show on the outdoor PCB 7-segment display, the PAR-series MA remote, or the AE-200 / AE-50 / EW-50 centralised controller. 6607 reports the address of the unit at fault; a 6607 with no source address points at the centralised-control bus path rather than a single unit.

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 the manual says to check
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.
Nexus iQ advantage: The platform watches the M-NET bus continuously and logs every dropout with the address and a timestamp, so an intermittent 6607 shows its pattern instead of clearing before you arrive. A 6607 with no source address shows in the Diagnostics view when a device stops answering.
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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 the manual says to check
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.
Nexus iQ advantage: 6608 is the response-timeout view of the same bus. The platform pins the drop to the address and the time it happened, so a wiring or terminating-resistor fault is diagnosed on the first visit.
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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 the manual says to check
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.
Nexus iQ advantage: The platform maps every M-NET address on the system, so a duplicate introduced during a board swap is caught before 6600 latches.
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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 the manual says to check
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.
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0403
Serial communication error / panel communication error
What it means
A serial communication error between the control board and the INV board on the compressor, and between the control board and the Fan board (detail code 1: control-to-INV; detail codes 5, 6: control-to-Fan). On indoor units it is a panel communication error when the unit cannot receive signals from the Auto filter cleaning unit for one minute.
What the manual says to check
Check the wiring between the control board and Fan board (CN2/CN2A/CN4/CN4A to CN80) and between the Fan board and INV board (CN82/CN83 to CN2/CN43).
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7102
Indoor unit count wrong or transmission down
What it means
The outdoor unit cannot get a valid count of connected indoor units. Either nothing is responding on the indoor/outdoor transmission line, or the count exceeds the outdoor model's limit. Often surfaces after a 6607 / CN40 fix on multi-outdoor systems, as the outdoor unit re-counts what is connected.
What the manual says to check
The indoor/outdoor transmission line at TB3 on the outdoor unit, or at the BC controller, is loose, broken, or unplugged. Most callouts on 7102 land here. Inspect terminations and recent cable runs.
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Pressure and Temperature Faults

High and low pressure trips and discharge-temperature protection. Confirm the trip against the gauges and the pressure sensor reading before anything else. On City Multi the pressures are read at the outdoor unit and the BC controller.

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 the manual says to check
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.
Nexus iQ advantage: High pressure and condenser temperature are trended daily, so a fouling coil or a blocked filter is caught before the 1302 high-pressure trip stops the compressor.
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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 the manual says to check
Compare the low-pressure sensor reading with gauge pressure (see manual section 8-5-3).
Nexus iQ advantage: Low pressure and superheat are trended, so a slow charge loss or a closed valve shows up before the 1301 low-pressure trip.
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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 the manual says to check
Evaluate and adjust the refrigerant charge (see manual section 6-9).
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 shows it in the Diagnostics view before the 1102 protection stop.
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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 the manual says to check
Evaluate and adjust the refrigerant charge (see manual section 6-9).
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Inverter, Compressor and Power Faults

The codes that make your stomach drop. These sit on the inverter and the compressor, the most expensive parts in the outdoor unit. Discharge the DC bus and verify before any connector work. Confirm the incoming three-phase supply first: an open phase (4102) or bus-voltage fault (4220) can look like an inverter failure.

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 the manual says to check
Check the inverter board error detection circuit, the compressor for ground fault and coil resistance, the inverter for damage, the setup conditions and the IGBT module (see manual section 8-10).
Nexus iQ advantage: Inverter current and module temperature are trended, so a stressed IPM is flagged before the 4250 overcurrent trip locks the system out.
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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 the manual says to check
Check the fan board error detection circuit and inverter (see manual sections 8-10-8 to 8-10-10).
Nexus iQ advantage: Heatsink temperature is trended against compressor frequency, so a blocked cooling path or a failing fan is caught before the 4230 overheat trip.
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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 the manual says to check
Check for a momentary power failure and that the phase-to-phase voltage (L1-L2, L2-L3, L1-L3) is above 342V.
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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 the manual says to check
Check the input voltage at the power supply terminal block TB1.
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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 the manual says to check
Check that the heat sink cooling air passage is not blocked.
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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 the manual says to check
Check the output wiring; confirm the U- and W-phase cables pass through CT12 and CT22 on the INV board.
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Sensor Faults

Thermistor and pressure-sensor faults on the indoor and outdoor units. These are a quick diagnosis: measure the sensor at the connector against the manual's resistance table, check the wiring, then replace the part.

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 the manual says to check
Compare the high-pressure sensor reading with gauge pressure (see manual section 8-5-1).
Nexus iQ advantage: The high-pressure sensor reading is trended, so a drifting 63HS1 is caught before it forces a protection stop or masks a real pressure problem.
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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 the manual says to check
Check the thermistor resistor: 0C 15 kOhm, 10C 9.7 kOhm, 20C 6.4 kOhm, 30C 4.3 kOhm, 40C 3.1 kOhm.
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5102
Pipe thermistor fault (TH22 / TH2)
What it means
A short or open of the indoor pipe thermistor (TH22) or the OA processing unit pipe thermistor (TH2). The preliminary code is 1217.
What the manual says to check
Check the thermistor resistor: 0C 15 kOhm, 10C 9.7 kOhm, 20C 6.4 kOhm, 30C 4.3 kOhm, 40C 3.1 kOhm.
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Drain and Water Faults

Drain pump, float switch and drain-sensor faults on the indoor units. A recurring 2502 is a drain pump error: on float-switch models the float has been held on, and on drain-sensor models the sensor's own self-heating can be misread as immersion.

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 the manual says to check
Check the drain pump for proper functioning.
Nexus iQ advantage: Drain pump behaviour and float-switch trips are logged, so a slow-draining pan is caught before 2502 stops the indoor unit and risks an overflow.
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2500
Drain sensor submergence
What it means
On models with a drain sensor or a float switch, immersion of the sensor detected in a mode other than Cool/Dry when the drain pump turns on, repeated over an hour, is treated as water leakage and 2500 appears.
What the manual says to check
Check for proper drainage (clogged drain pump or piping, or backflow from other units).
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2503
Drain sensor (Thd) fault
What it means
An open or short circuit of the drain thermistor detected for 30 seconds during Cool/Dry or with the drain pump running (short 90C [194F] or above; open -20C [-4F] or below).
What the manual says to check
Reinsert the connector, restart and check operation.
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Tired of chasing City Multi codes?

Nexus iQ monitors discharge temperature, pressures, compressor current and M-NET communication health 24/7. It catches the patterns that lead to fault codes before they trip.

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What Monitoring Catches That Fault Codes Miss

Fault codes are reactive. By the time a code appears, the fault has already happened. Every system generates dozens of parameters that change continuously: discharge temperature, high and low pressure, compressor current and frequency, and M-NET communication health. In a healthy system these hold consistent relationships. When something starts to degrade, a slow leak, a fouling coil, a communication 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. When the patterns diverge you get a diagnostic insight, not a lockout.

ScenarioWithout MonitoringWith Nexus iQ
Slow refrigerant leak1301 low-pressure fault on the coldest morning. Emergency callout.Low-pressure trend flagged weeks earlier. Leak found during scheduled service.
Condenser fouling1302 high-pressure fault on the hottest day.High-pressure trend rising. Coil clean scheduled before summer peak.
Compressor stress building1102 discharge-temperature fault, or a 4250 inverter overcurrent. Compressor risk.Discharge and current trends flagged months earlier. Planned repair.
Communication intermittentRandom 6607 / 6608 faults. Multiple callouts, no pattern found.M-NET dropouts logged with address and timestamp. Pattern identified on the first visit.
Discharge temperature climbing1102 discharge-temperature protection stop.Discharge trend against frequency flagged. Charge or valve checked before the stop.

The most expensive City Multi fault code is the one that trips on a Friday afternoon in July. Monitoring does not prevent failures, it prevents surprises.

Quick Reference: Every Trane US VRF Code in This Guide

Use this searchable table to find any City Multi code used on Trane US VRF. Type a code (e.g. 1302), a keyword (e.g. pressure), or a category. Click any highlighted code to jump to its detail above.

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 Trane US VRF 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 City Multi M-NET bus. It reads every data point the system generates, temperatures, pressures, currents, frequencies, 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.

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Sources

The code meanings in this guide are the Mitsubishi Electric City Multi self-diagnosis codes, drawn from the following Mitsubishi Electric North America service document. The same 4-digit M-NET codes apply to Trane VRF supplied through METUS.

Trane US VRF fault codes: frequently asked questions

What does Trane error code 6607 mean?

6607 is an M-NET no-acknowledgement (No ACK) error: a device on the M-NET bus did not acknowledge a transmission within the expected window. The address shown alongside 6607 identifies the unit that failed to respond, which is where to start. If 6607 shows with no source address, suspect the centralised-control bus path rather than a single unit. Check the M-NET wiring, polarity, terminating resistors and power. This applies to Trane VRF sold in the United States (Mitsubishi City Multi).

Are Trane's US VRF codes the same as Mitsubishi City Multi?

Yes. Trane VRF in the United States is Mitsubishi Electric City Multi supplied through METUS, so it uses the same 4-digit M-NET self-diagnosis codes. Trane VRF sold in Australia and Europe is a different platform with a different code set.

Can I monitor Trane VRF fault codes remotely?

Yes. The AirNexus Nexus iQ platform connects to Trane / City Multi VRF 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.

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