MHI vs Mitsubishi Electric — Know the Difference
This is the single most common point of confusion in Australian HVAC. Mitsubishi Heavy Industries (MHI) and Mitsubishi Electric are separate companies under the broader Mitsubishi Group. Different factories, different engineering teams, different VRF systems, different fault codes, different communication protocols. They are not interchangeable.
- MHI HVAC is marketed as “Mitsubishi Heavy Industries Thermal Systems” — the three-diamond logo with “HEAVY INDUSTRIES” text. VRF systems: KX series (KX6, KXR6, KXZ), FDC series outdoor units, FDTC/FDT indoor units
- Mitsubishi Electric HVAC uses City Multi (PUHY, PURY outdoor units) with the M-NET communication protocol
- MHI uses its own proprietary serial communication bus — it is NOT M-NET
- If your outdoor unit model starts with FDC — it’s MHI. This guide
- If your outdoor unit model starts with PUHY or PURY — it’s Mitsubishi Electric. Different guide
Quick check: Look at the model number on your outdoor unit. FDC = MHI (this guide). PUHY/PURY = Mitsubishi Electric (different guide). Getting this wrong means looking up the wrong fault codes.
“Not Mitsubishi Electric. Not the same fault codes. Not the same protocol. Know the difference.”
Looking for Mitsubishi Electric City Multi codes? See our Mitsubishi Electric VRF Error Code Guide →
Communication Faults
Communication faults on MHI systems are common during commissioning and can be intermittent in operation. MHI uses a polarity-sensitive serial bus — unlike Mitsubishi Electric’s M-NET, getting the polarity wrong on even one indoor unit will cause communication errors across the system.
E28
Communication error between outdoor and indoor units
What it means
The outdoor unit has lost serial communication with one or more indoor units. The system will typically shut down the affected indoor units while others continue to operate.
What to check first
Check the communication wiring between outdoor and indoor units. Verify polarity — MHI IS polarity-sensitive unlike Mitsubishi Electric. Check for damaged cables, water ingress at junction boxes, and loose terminals at both ends. Measure voltage on the communication bus — it should show a clean signal.
Common cause
Reversed polarity on one indoor unit during installation. Damaged communication wire run through during construction. Water ingress at an outdoor junction box.
Nexus iQ advantage: Communication drops are intermittent — they happen once, the system recovers, nobody notices. Until it becomes persistent. Nexus iQ logs every communication timeout, so you can see the pattern before it becomes a problem.
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E29
Communication error between outdoor units
What it means
In a multi-outdoor-unit system, inter-unit communication has failed. The sub unit cannot communicate with the master unit.
What to check first
Check the inter-unit communication cable between outdoor units. Verify master/sub addressing via DIP switches on each outdoor unit PCB. Check for duplicate addresses. Inspect cable for physical damage.
Common cause
DIP switch misconfiguration after a PCB replacement. Cable damage from rooftop maintenance work.
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E31
Communication error with remote controller
What it means
The wired remote controller has lost communication with the indoor unit it’s connected to.
What to check first
Check the 2-wire connection from the controller to the indoor unit PCB. Verify the address setting on the controller matches the indoor unit. Power cycle the controller by disconnecting and reconnecting.
Common cause
Loose terminal at the indoor unit PCB connector. Incorrect address setting after a controller replacement.
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E32
Address setting error
What it means
An indoor unit address is not set or duplicates another unit on the same system.
What to check first
Verify all indoor unit addresses are unique. Re-address via DIP switches on the indoor unit PCB or through the remote controller setup menu. Check that no two units share the same address number.
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Refrigerant & Pressure Faults
Pressure faults are the system telling you something is fundamentally wrong with the refrigerant circuit. They’re never random — there’s always a cause, and ignoring them leads to compressor damage.
E01
High pressure fault
What it means
Discharge pressure has exceeded the high pressure limit. The compressor will shut down to protect itself.
What to check first
Check the condenser coil for blockage — leaves, debris, cottonwood seeds. Verify condenser fan operation (all fans spinning at correct speed). Check refrigerant charge — overcharge causes high pressure. Check outdoor ambient temperature — extreme heat days can push borderline systems over the limit.
Common cause
Dirty condenser coil. Condenser fan motor failure. Refrigerant overcharge from incorrect commissioning calculations.
Nexus iQ advantage: A recurring E01 that goes unresolved leads to compressor stress. Each high pressure event shortens compressor life. Nexus iQ tracks high pressure events and their frequency — so you can see the pattern before it becomes chronic.
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E02
Anti-freeze protection (indoor coil)
What it means
Indoor coil temperature has dropped too low. The system has activated freeze protection to prevent ice buildup on the evaporator.
What to check first
Check indoor fan operation — is the fan running at the correct speed? Check the air filter — a blocked filter restricts airflow and drops coil temperature. Check the evaporator coil for dirt buildup. Verify refrigerant charge — undercharge causes low evaporator temperature.
Common cause
Blocked air filter. Dirty evaporator coil. Low refrigerant charge from a slow leak.
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E03
Low pressure fault
What it means
Suction pressure has dropped below the minimum threshold. The system has shut down the compressor to prevent damage from running at low suction pressure.
What to check first
Check refrigerant charge — most likely undercharged. Look for restrictions in the liquid line: blocked filter drier, kinked pipe, partially closed service valve. Check expansion valve operation. On long pipe runs, check for oil logging in the piping.
Common cause
Slow refrigerant leak at a flare connection. Blocked filter drier. Incorrect charge calculation on initial commissioning.
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E07
Refrigerant overcharge
What it means
The system has detected excessive refrigerant charge based on operating parameters.
What to check first
Check subcooling values at the outdoor unit liquid service valve. If subcooling is above 15K, the system is overcharged. Recover excess refrigerant to the correct charge. Re-check the additional charge calculation based on pipe length.
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E09
High discharge temperature
What it means
Compressor discharge temperature has exceeded the safe limit. This is one of the most important fault codes on any VRF system — it indicates the compressor is under stress.
What to check first
Check refrigerant charge — undercharge is the most common cause of high discharge temperature. Check condenser airflow. Verify expansion valve operation — a stuck valve causes poor superheat control. Check for non-condensables (air) in the system from a poor vacuum during commissioning.
Common cause
Low refrigerant charge. Poor condenser airflow. Non-condensables in the refrigerant circuit.
Nexus iQ advantage: Discharge temperature is the most important trending parameter on any VRF system. Nexus iQ tracks it continuously — a gradual rise over weeks means the system is heading toward this fault long before the code triggers.
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“Discharge temperature is the single most important trending parameter. Watch it, and you’ll predict 80% of compressor faults.”
Compressor & Inverter Faults
These are the expensive ones. A compressor replacement on an MHI KX system runs $10,000–$18,000 including labour. An inverter board is $2,500–$3,500. Every fault code in this section is a warning that something needs attention before it becomes a five-figure repair.
E04
Compressor overcurrent
What it means
The inverter has detected overcurrent on the compressor motor. The system has shut down to protect the compressor and inverter.
What to check first
Check compressor insulation resistance with a megger test — minimum 1MΩ to ground. Check for liquid slugging by verifying superheat at the compressor suction — if superheat is below 3K, liquid refrigerant is reaching the compressor. Check incoming voltage at the outdoor unit terminals under load.
Common cause
Compressor winding degradation. Liquid slugging from incorrect charge or failed expansion valve. Voltage sag under load from undersized cabling.
Nexus iQ advantage: Compressor replacement on an MHI KX system: $10,000–$18,000. Catching overcurrent trends early — before they become persistent — prevents catastrophic failure.
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E05
Inverter fault
What it means
The inverter board has detected an internal error. This could be a transient fault from a power event or a permanent hardware failure.
What to check first
Check for voltage spikes or power quality issues — install a power logger if the fault is intermittent. Inspect the inverter board for signs of damage: burn marks, bulging capacitors, corrosion. Power cycle the system (30 seconds off, then restart). If the fault persists after power cycle, inverter board replacement is required.
Common cause
Power surge or lightning strike damage. Capacitor degradation from age and heat cycling.
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E06
Compressor motor lock / startup failure
What it means
The compressor failed to start or is mechanically locked. The inverter attempted to start the compressor but could not achieve rotation.
What to check first
Check compressor winding resistance between all three phases — readings should be balanced and within the manufacturer’s specification. Check for mechanical seizure by trying to start in a different mode. Verify the crankcase heater is functional and has been energised for at least 12 hours before startup.
Common cause
Liquid refrigerant migration to the crankcase during long off-periods, causing liquid slugging on startup. Failed crankcase heater.
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E10
Compressor over-speed
What it means
Compressor frequency exceeded the maximum limit. The system has shut down to prevent mechanical damage.
What to check first
Check refrigerant charge. Check for restrictions in the refrigerant circuit that could cause the compressor to run at higher frequency to compensate. Verify inverter board output frequency against the expected range.
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E13
Inverter overcurrent (instantaneous)
What it means
A short-duration current spike was detected on the inverter output. This is different from E04 (sustained overcurrent) — E13 is a momentary spike.
What to check first
Check compressor winding insulation resistance. Check wiring between the inverter and compressor for short circuits or loose connections. Check for moisture in the electrical compartment that could cause arcing.
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E14
Inverter overtemperature
What it means
The inverter heatsink temperature has exceeded the safe limit. The system has shut down to protect the inverter power modules from thermal damage.
What to check first
Check the heatsink for dust buildup — a compressed air clean may be all that’s needed. Verify the heatsink fan operation. Check refrigerant charge — on MHI systems the inverter is often refrigerant-cooled, so low charge means poor inverter cooling.
Common cause
Dust accumulation on the heatsink fins. Failed heatsink cooling fan. Low refrigerant charge affecting inverter cooling.
Nexus iQ advantage: A $200 heatsink cleaning prevents a $3,500 inverter board replacement. Monitoring catches the temperature trend before the fault code triggers.
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“A $200 heatsink cleaning prevents a $3,500 inverter board replacement.”
Tired of chasing MHI fault codes?
Nexus iQ monitors discharge temperature, compressor frequency, and communication health 24/7 — so you catch problems before they become fault codes.
See How It WorksSensor Faults
Sensor faults are usually straightforward — a thermistor has failed or its wiring is damaged. They’re cheap to fix but important to diagnose correctly, because a faulty sensor can cause the system to make bad decisions about refrigerant control.
E15
Outdoor air temperature sensor fault
What it means
The outdoor ambient temperature sensor is reading out of range or has an open circuit.
What to check first
Measure thermistor resistance and compare against the MHI resistance curve in the service manual. Check the connector at the outdoor unit PCB. Replace if open circuit or readings are erratic.
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E16
Discharge temperature sensor fault
What it means
The compressor discharge temperature sensor has failed or is reading out of range.
What to check first
Measure thermistor resistance. Check sensor placement on the discharge pipe — it must have good thermal contact. Check wiring back to the PCB.
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E17
Suction temperature sensor fault
What it means
The suction temperature sensor is reading out of range or has failed.
What to check first
Measure thermistor resistance. Check placement on the suction pipe. Verify wiring continuity.
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E18
Heat exchanger temperature sensor fault (outdoor)
What it means
The condenser coil temperature sensor has failed or is reading abnormally.
What to check first
Measure thermistor resistance. Check placement on the condenser coil. Replace if open circuit.
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E19
Indoor room temperature sensor fault
What it means
The return air temperature sensor on the indoor unit is reading out of range or has an open/short circuit.
What to check first
Measure thermistor resistance — typically 10kΩ at 25°C for MHI sensors. Check the connector on the indoor unit PCB. Check for physical damage to the sensor or its wiring.
Common cause
Sensor wire pinched during installation or ceiling work. Sensor physically displaced during filter cleaning.
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E20
Indoor heat exchanger temperature sensor fault
What it means
The indoor coil temperature sensor has failed or is reading abnormally.
What to check first
Measure thermistor resistance. Check placement on the evaporator coil. Replace if open circuit or readings don’t match the resistance curve.
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Outdoor Unit & System Faults
E21
Four-way valve fault
What it means
The reversing valve has not switched correctly between heating and cooling modes.
What to check first
Check the 4-way valve coil for continuity. Listen for the valve switching sound when changing modes. Compare discharge and suction temperatures before and after a mode change — if they don’t swap, the valve is stuck.
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E22
Condenser fan motor fault
What it means
The outdoor fan motor has failed or is drawing excessive current.
What to check first
Check fan motor winding resistance. Check for physical obstructions — debris, birds’ nests. Spin the fan by hand to check bearings. Check the motor connector on the PCB.
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E23
External input protection
What it means
An external safety device wired to the outdoor unit (flow switch, external high pressure cutout, smoke detector interlock, etc.) has triggered.
What to check first
Identify all external safety devices wired to the outdoor unit’s external input terminals. Check which device has triggered. Reset if safe to do so. If no external devices are wired, check for a shorted or open input terminal.
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E24
Power supply phase fault
What it means
Incorrect phase sequence or phase loss detected at the outdoor unit. The system will not start until correct three-phase power is restored.
What to check first
Check all three phases at the outdoor unit terminals with a multimeter. Verify phase sequence with a phase rotation meter. Check for single-phasing (one phase dropped) — measure voltage between all phase pairs.
Common cause
Power outage recovery with incorrect phase sequence. Loose phase connection at the isolator or switchboard. Blown fuse on one phase.
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E25
Oil recovery fault
What it means
The oil recovery cycle has failed — compressor oil is not returning from the refrigerant circuit back to the compressor sump.
What to check first
Check oil level in the sight glass. Verify oil recovery piping is not blocked. Check refrigerant charge — incorrect charge affects oil circulation. On systems with long pipe runs, check for oil traps in vertical risers.
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E26
Capacity control fault
What it means
The system cannot achieve the requested heating or cooling capacity.
What to check first
Check that all outdoor units in a multi-unit system are running. Check for indoor units that are locked out with other fault codes. Verify refrigerant charge — undercharge limits system capacity.
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Indoor Unit Faults
E33
Indoor fan motor fault
What it means
The indoor unit fan motor has failed or is drawing excessive current.
What to check first
Check motor winding resistance. Check for physical obstructions in the fan scroll. Check bearings by spinning the fan by hand. Check the motor connector on the indoor PCB.
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E34
Drain pan overflow / Float switch fault
What it means
The condensate drain pan is full or the float switch has activated, indicating a drainage problem.
What to check first
Check the condensate drain line for blockage — use compressed air or a wet vacuum to clear. Check the drain pan for standing water. Verify float switch operation by manually lifting the float. Clean the drain pan if algae or dirt has accumulated.
Common cause
Blocked drain line from algae growth, dirt, or insulation debris. Kinked drain hose. Drain pump failure (if fitted).
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E35
Indoor PCB fault
What it means
The indoor unit control board has detected an internal error that it cannot recover from.
What to check first
Power cycle the indoor unit by switching off its isolator for 30 seconds. Check for voltage irregularities at the indoor unit power supply. If the fault persists after power cycling, PCB replacement is required.
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What Monitoring Catches
MHI error codes tell you what’s broken. Monitoring tells you what’s about to break. The difference is the gap between a $200 preventive service visit and a $15,000 emergency compressor replacement.
- Discharge temp trending up = compressor stress developing, weeks before E09 triggers
- Communication drops happening intermittently = wiring degradation before E28 becomes persistent
- High pressure events increasing in frequency = condenser degradation before E01 becomes chronic
- Inverter temperature creeping up = dust buildup on the heatsink before E14 triggers
- COP degrading gradually = efficiency loss that generates no fault code at all
| WITHOUT MONITORING | WITH NEXUS iQ |
|---|---|
| Fault code triggers → emergency callout | Trend detected → scheduled service |
| No history of what happened before the fault | Full data: temps, pressures, Hz, runtime |
| Guessing which unit in a multi-unit MHI system | Nexus pinpoints the exact unit |
| $2,000–$5,000 emergency callout | $200 preventive service |
| Compressor fails → $10,000–$18,000 replacement | Compressor stress caught early |
The most expensive fault code is the one you could have prevented. Monitoring doesn’t stop failures — it stops surprises.
Quick Reference Table
Use this searchable table to quickly find any MHI VRF fault code. Type a code (e.g., E01) or a keyword (e.g., pressure) to filter the results.
| Code | Category | Description | Severity |
|---|---|---|---|
| E01 | Refrigerant | High pressure fault | Critical |
| E02 | Refrigerant | Anti-freeze protection (indoor coil) | Warning |
| E03 | Refrigerant | Low pressure fault | Critical |
| E04 | Compressor | Compressor overcurrent | Critical |
| E05 | Compressor | Inverter fault | Critical |
| E06 | Compressor | Compressor motor lock / startup failure | Critical |
| E07 | Refrigerant | Refrigerant overcharge | Warning |
| E09 | Compressor | High discharge temperature | Critical |
| E10 | Compressor | Compressor over-speed | Critical |
| E13 | Compressor | Inverter overcurrent (instantaneous) | Critical |
| E14 | Compressor | Inverter overtemperature | Critical |
| E15 | Sensor | Outdoor air temperature sensor fault | Info |
| E16 | Sensor | Discharge temperature sensor fault | Warning |
| E17 | Sensor | Suction temperature sensor fault | Warning |
| E18 | Sensor | Heat exchanger temperature sensor (outdoor) | Warning |
| E19 | Sensor | Indoor room temperature sensor fault | Warning |
| E20 | Sensor | Indoor heat exchanger temperature sensor | Warning |
| E21 | System | Four-way valve fault | Warning |
| E22 | System | Condenser fan motor fault | Warning |
| E23 | System | External input protection | Warning |
| E24 | System | Power supply phase fault | Critical |
| E25 | System | Oil recovery fault | Warning |
| E26 | System | Capacity control fault | Warning |
| E28 | Communication | Communication error — outdoor to indoor | Critical |
| E29 | Communication | Communication error — between outdoor units | Warning |
| E31 | Communication | Communication error — remote controller | Warning |
| E32 | Communication | Address setting error | Info |
| E33 | Indoor | Indoor fan motor fault | Warning |
| E34 | Indoor | Drain pan overflow / Float switch | Warning |
| E35 | Indoor | Indoor PCB fault | Critical |
Getting Started
Knowing what a fault code means is step one. Preventing it from happening is step two. Here’s how to move from reactive to predictive maintenance on your MHI VRF systems.
1. Book a Demo
See Nexus iQ monitoring a live MHI VRF system — real fault history, real diagnostic charts, real trending data. We’ll show you exactly how the platform detects developing issues before they become fault codes.
2. Connect Your System
A Nexus 32 controller connects directly to MHI KX and FDC series communication buses. Installation takes under a day, requires no downtime, and doesn’t affect the operation of your existing system. The controller reads every data point the MHI system generates and streams it to Nexus iQ in real time.
3. Stop Guessing
Within 24 hours of installation, you’ll have a complete diagnostic view of every outdoor and indoor unit. Within two weeks, baseline patterns are established. Within a month, you’ll start receiving predictive insights — the subtle trend changes that no fault code would ever catch.