Mitsubishi Heavy Industries Error Codes

If your Mitsubishi Heavy Industries air conditioning unit has flagged a fault, the error code on the wired remote (or the flashing LEDs on the indoor unit) is the fastest way to work out what is wrong. The code tells you which sensor, board or pressure switch has triggered the alarm, so you can either fix it yourself or brief an engineer properly before they arrive on site.

This guide covers every Mitsubishi Heavy Industries (MHI) error code we see in the field across the FDT cassette, FDU ducted, FDK wall, FDE ceiling suspended, FDUM concealed and KX VRF ranges. Each code is paired with what it actually means, the most likely cause, and what to do about it. We have been servicing MHI commercial AC across London and the Home Counties for over twenty years, so the troubleshooting notes are based on what fixes the unit, not just what the manual says.

How to Read Mitsubishi Heavy Industries Error Codes

MHI splits faults into two groups. E codes are hardware or communication errors detected by the control board. P codes are protection codes, where the unit has shut itself down to avoid damage (high pressure, overheat, current cut-out). Both appear on the RC-E5, RCH-E3 or RC-EX wired remote controllers, and on the seven-segment display of the outdoor PCB on KX VRF systems.

If you have no remote, check the operation lamp on the indoor unit. A continuous flash pattern (for example three flashes, pause, three flashes) corresponds to the second digit of the code. The Run lamp flashing while the Timer lamp is off generally indicates an indoor fault. Both lamps flashing together points to an outdoor unit fault.

Mitsubishi Heavy Industries Communication Error Codes (E1 to E12)

These codes appear when the indoor unit, outdoor unit and remote controller cannot talk to each other properly. Roughly 60% of MHI callouts we attend with a communication error turn out to be a wiring issue rather than a failed board.

Code	Meaning	Likely Cause	What to Do
E1	Remote control communication error	The wired remote has lost contact with the indoor PCB. Usually a damaged X/Y signal cable, a loose terminal, or a remote that has been wired with reversed polarity.	Check the X/Y terminals on both the remote and the indoor unit. The cable must be unscreened twin-core and under 600m. Power the unit off at the isolator for two minutes before retesting. If the remote is dead, swap with a known working RC-E5 to confirm before replacing.
E2	Duplicate indoor unit address	Two indoor units on the same VRF group have been given the same address by the DIP switches on the PCB.	Open each indoor PCB and check the address rotary switches (SW1, SW2). Each indoor unit needs a unique address within the group. After changing, power off for at least three minutes before powering back on.
E3	Outdoor unit signal line error	The signal cable between indoor and outdoor units is broken, shorted, or has water ingress at the terminal block.	Inspect the interconnecting cable for damage, especially where it passes through walls or external trunking. Check the outdoor terminal block for moisture and corrosion. Megger test the cable if the run is long.
E5	Communication error during operation	Connection was working at start-up but has dropped out, often caused by electrical interference, a failing transformer on the indoor PCB, or a marginal cable.	Check the indoor PCB transformer output (should be around 22V AC). Look for high-current cables running parallel to the signal cable, as these can induce interference. Replace the indoor PCB if voltage is correct but the fault persists.
E6	Indoor heat exchanger temperature thermistor abnormality (Thi-R)	The Thi-R sensor on the indoor coil has failed, become disconnected, or is reading out of range (open or short circuit).	Disconnect the Thi-R thermistor at the PCB and measure resistance. At 25°C it should read around 10 kΩ. Replace if readings are infinite or zero. Check the connector is fully seated.
E7	Indoor return air temperature thermistor abnormality (Thi-A)	The Thi-A sensor measuring return air at the indoor unit has failed or its wiring is damaged.	Same procedure as E6. The Thi-A is usually clipped near the return air grille, so check it has not been pulled loose during a filter clean.
E9	Drain water level abnormality (unloading)	The condensate drain pump cannot clear water from the drain tray, or the float switch has triggered.	Remove the drain pump and clean any sludge from the impeller and trap. Check the condensate pipe falls correctly with no sags. Test the float switch with a multimeter for continuity. A blocked condensate is the number one cause we see.
E10	Excessive number of indoor units controlled by one remote	More than 16 indoor units have been wired into a single group controller.	Reduce the group size to 16 or fewer indoor units, or split into multiple control groups using a central controller.
E12	Address setting error with mixed setting method	Both automatic and manual address setting have been used in the same VRF system.	Choose one method. For most installations, automatic addressing is simpler. Set all DIP switches consistently and re-initialise the system.

Mitsubishi Heavy Industries Sensor Error Codes

Sensor errors are some of the easiest faults to fix because the thermistors themselves are inexpensive and clip in and out. Always check the connector and wiring before condemning the sensor.

Code	Meaning	Likely Cause	What to Do
E28	Remote control temperature thermistor abnormality (ThC)	The temperature sensor inside the wired remote controller has failed.	Replace the remote, or set the controller to use the indoor unit return air sensor instead via the function settings menu.
E37	Outdoor heat exchanger or sub-cooling thermistor abnormality (Tho-R, Tho-SC, Tho-H)	One or more outdoor coil thermistors are reading out of range. Often water ingress at the connector after heavy rain.	Open the outdoor PCB cover and inspect the thermistor connectors for green corrosion. Dry, clean and re-grease with dielectric grease. Replace any thermistor that does not read 10 kΩ at 25°C.
E38	Outdoor air temperature thermistor abnormality (Tho-A)	The ambient air sensor on the outdoor unit has failed or its wiring is damaged.	Located near the air intake grille on the outdoor unit. Check it has not been pulled by birds, vermin or installation debris. Test resistance and replace if needed.
E39	Discharge pipe temperature thermistor abnormality (Tho-D1, D2)	The thermistor on the compressor discharge line is reading open or short.	Access the thermistor through the outdoor service panel. These are usually strapped to the discharge pipe with metal clips. Confirm the clip is tight and the sensor body is fully against the pipe.
E53	Suction pipe temperature thermistor abnormality (Tho-S)	Suction line thermistor has failed.	Same diagnostic as E39 but on the suction line. Replace if open or short.
E54	High or low pressure sensor abnormality (PSH, PSL)	The pressure transducer on either the high or low side has failed or its 5V supply is missing.	Check the 5V DC supply at the PCB connector. If the supply is correct, replace the pressure sensor. Note that some MHI models use a 0–5V transducer rather than a switch, so a multimeter on resistance mode will not test these.
E55	Compressor under-dome thermistor abnormality (Tho-C1, C2)	The thermistor inside the compressor dome (used for liquid back protection) has failed.	This thermistor is part of the compressor assembly on most models and cannot be replaced separately. Confirm with a resistance test before deciding to replace the compressor.
E56	Power transistor temperature thermistor abnormality (Tho-P1, Tho-P2)	The thermistor monitoring the IPM (inverter power module) heat sink has failed.	Located on the inverter PCB. Replace the thermistor, or the inverter PCB if the sensor is integrated.

Mitsubishi Heavy Industries Fan, Motor and Compressor Error Codes

Mechanical faults tend to be the most expensive to repair if left, because a failing fan can cause a high pressure trip which can damage the compressor. Address these quickly.

Code	Meaning	Likely Cause	What to Do
E16	Indoor fan motor abnormality (FDT, FDK series)	The DC fan motor is drawing too much current, has failed, or has a damaged Hall sensor feedback wire.	Power off and turn the fan by hand. If it grinds or scrapes, the bearing has failed and the motor must be replaced. If it spins freely, check the feedback connector at the indoor PCB.
E19	Indoor unit operation control unload motor control mode abnormality	Control issue between the indoor PCB and the unloading mechanism.	Power cycle the unit. If the fault persists, the indoor PCB usually needs replacing.
E48	Outdoor DC fan motor abnormality	Outdoor fan motor has failed, the propeller is obstructed, or the inverter section driving the fan has failed.	Check for ice, leaves or debris jamming the propeller, especially after winter. Spin the fan by hand to test for free rotation. If clear, the motor or fan inverter PCB needs replacing.
E58	Compressor with loss of synchronisation	The inverter can no longer drive the compressor, often due to refrigerant overload, undercharge, or a failing IPM.	Check refrigerant charge against the data plate. Verify high and low pressures match the operating envelope. If pressures are normal, the inverter PCB or compressor windings have likely failed.
E59	Compressor start failure (CM1, CM2)	The compressor has failed to start three times in a row, often because it is liquid locked, electrically failed, or the inverter PCB cannot deliver start torque.	Megger test the compressor windings to ground (must be greater than 10 MΩ). Test winding resistance between phases for balance. If electrically sound, check the inverter PCB output.
E60	Compressor rotor position detection failure	The inverter cannot detect the rotor position of the BLDC compressor, indicating a failing IPM or compressor.	This usually means the inverter PCB or compressor needs replacement. Confirm by swapping the inverter PCB first if available, as this is the cheaper fix.

Mitsubishi Heavy Industries Pressure and Power Error Codes

Pressure and power faults often point at the wider system rather than the AC unit itself. Always rule out airflow, voltage and refrigerant before replacing parts.

Code	Meaning	Likely Cause	What to Do
E30	Incompatible connection of indoor and outdoor unit	The indoor and outdoor unit models are not compatible, or the firmware versions cannot communicate.	Cross-check both model numbers against the MHI compatibility table. If the system was working previously, check whether a board has been replaced with the wrong part number.
E31	Duplicated outdoor unit address	Two outdoor units in the same VRF system have the same address.	Reset the rotary switches on each outdoor PCB to give every unit a unique address.
E32	Open L3 phase in the primary side power supply	One of the three phases is missing at the outdoor unit, often after a phase failure at the supply, a blown fuse, or a loose connection in the isolator.	Check voltage between L1, L2 and L3 at the outdoor unit isolator. All three should read 400V phase to phase. If a phase is missing, isolate and check the supply chain back to the distribution board.
E36	Discharge pipe temperature error or liquid overflow	The discharge pipe is running too hot (typically over 130°C) due to undercharge, or refrigerant has migrated to the compressor as liquid.	Check the refrigerant charge and superheat. If undercharged, leak test, repair and recharge by weight. If overcharged, recover excess.
E40	High pressure abnormality (63H1-1, 63H1-2 active)	The high pressure switch has tripped at around 4.15 MPa. Causes include blocked outdoor coil, failing outdoor fan, overcharge, or non-condensable gases in the system.	Clean the outdoor coil. Confirm the outdoor fan is running on full. Check the suction and discharge pressures against the data plate. Recover, evacuate and recharge by weight if non-condensables are suspected.
E42	Compressor current cut-out (CM1, CM2)	The CT has detected an over-current event on the compressor, usually caused by high pressure operation, low voltage, or a failing compressor.	Check supply voltage under load. Inspect the CT clip-on around the compressor cable for damage. Confirm the system is not running at high pressure.
E43	Over-connected indoor units or excessive total capacity	The total connected indoor capacity exceeds the outdoor unit rating, or too many indoor units are connected.	Recalculate the indoor connected capacity. MHI VRF allows up to 130% connection ratio in most cases. Above this, indoor units must be removed or downsized.
E45	Communication error between inverter PCB and outdoor control PCB	The internal cable between the main outdoor PCB and the inverter PCB is loose, damaged, or one of the boards has failed.	Open the outdoor control box and reseat the ribbon cable between PCBs. Check for rodent damage to the cable. Replace the cable or PCB as needed.
E46	Mixed address setting methods on the same network	Same root cause as E12 but flagged at outdoor level.	Standardise to either auto or manual addressing across the whole VRF network.
E49	Low pressure abnormality	The low pressure switch has tripped at around 0.05 MPa. Caused by a refrigerant leak, restricted EEV, blocked filter drier, or a failing pressure sensor.	Run a full leak check with electronic leak detector and nitrogen pressure test. Replace the filter drier if the system has been opened. Test the EEV coil resistance.
E61	Communication error between main and backup outdoor units	In multi-outdoor VRF systems, the master and slave units have lost communication.	Check the inter-unit communication cable. Confirm the master/slave DIP switches are set correctly. Power off all outdoor units for five minutes before restarting.
E63	Emergency stop	An external safety input (fire alarm, BMS, manual stop) has triggered the emergency stop terminal on the outdoor PCB.	Check the external interlock signal. The fault clears once the external input is restored. Check fire panel status and BMS commands.

Mitsubishi Heavy Industries P-Codes (Protection Codes)

P-codes are protective shutdowns rather than hardware failures. The unit has stopped to protect itself, and once the underlying condition is corrected, the unit will restart.

Code	Meaning	Likely Cause	What to Do
P1	High pressure protection	System pressure has approached the high-pressure cut-out threshold but not tripped E40 yet.	Same diagnostic path as E40. Clean the condenser coil and check the outdoor fan first.
P2	Low pressure protection	System has detected approaching low pressure trip.	Check refrigerant charge, leak test, and inspect the EEV.
P3	Discharge temperature protection	Discharge pipe is running hot but below the E36 trip point.	Check superheat and refrigerant charge. Often points to a slowly leaking system.
P5	Compressor over-current protection	Compressor is drawing more current than expected but below cut-out.	Check supply voltage, system pressures, and condenser airflow.
P9	Indoor heat exchanger frost or overheat protection	Coil temperature is out of normal range for the operating mode (frosted in cooling, too hot in heating).	Check filters, fan operation, and refrigerant charge. A blocked filter is the most common cause.

When to Call a Mitsubishi Heavy Industries Engineer

You can usually clear E1, E5, E9, E63 and most P-codes yourself by checking wiring connections, clearing the drain, or restoring power. Anything involving pressure faults, compressor codes, or inverter PCB faults needs an F-Gas certified engineer with the right MHI service tools, including the SLA-3IE service checker for KX VRF systems.

If your unit is throwing a code that is not listed here, or the same code keeps returning after a fix, it is worth getting a proper service rather than guessing at parts. Many MHI faults look similar on the remote but have completely different root causes. We see plenty of jobs where the wrong PCB has been replaced before someone calls us in.

Mitsubishi Heavy Industries Error Codes FAQ

What does E40 mean on a Mitsubishi Heavy AC?

E40 is a high pressure abnormality. The 63H1 high-pressure switch on the discharge line has tripped at around 4.15 MPa. The four most common causes are a dirty outdoor coil, a failing outdoor fan, an overcharged system, or non-condensable gases. Clean the coil and check the fan first because those are the cheap fixes.

How do I reset a Mitsubishi Heavy Industries error code?

Switch the unit off at the wired remote, then off at the mains isolator for at least three minutes. Switch the mains back on, wait for the indoor unit to fully boot, then switch on at the remote. If the fault was a transient (like a momentary low voltage event), it will clear. If the underlying fault is still present, the same code will return within a few minutes of operation.

Where is the Mitsubishi Heavy error code displayed?

On the wired remote (RC-E5, RCH-E3, or RC-EX), the error code shows on the LCD screen with the unit address. On the indoor unit itself, watch the operation lamp flash pattern. On KX VRF outdoor units, the seven-segment display on the main outdoor PCB shows the active error code.

What is the difference between MHI E codes and P codes?

E codes are hardware or communication errors, where something has failed or cannot be detected. P codes are protection events, where the unit has shut down to protect itself from damage but no part has actually failed yet. Clear the underlying condition that triggered the P code and the unit returns to normal operation.

How much does it cost to fix a Mitsubishi Heavy AC error code?

Simple sensor replacements (E6, E7, E28, E38) typically run £180 to £350 including labour. Drain pump replacements (E9) run £250 to £400. PCB replacements (E45, E60) typically run £500 to £900 depending on model. Compressor failures on KX VRF systems start at around £1,800. Always get a proper diagnosis before approving parts.

Why does my MHI unit keep throwing the same error code?

Recurring codes nearly always mean the symptom has been treated rather than the cause. For example, a recurring E40 is rarely the high pressure switch itself, it is usually a dirty coil, a failing fan, or an overcharged system. A recurring E59 is rarely the compressor, it is usually the inverter PCB or supply voltage.

Can I fix a Mitsubishi Heavy error code myself?

Anything involving wiring checks, drain cleaning, filter cleaning, or power cycling is fair game for an in-house facilities team. Anything involving refrigerant, pressure switches, compressors, or inverter boards must be carried out by an F-Gas certified engineer in the UK. Working on a sealed refrigerant system without F-Gas certification is illegal.

Get Your Mitsubishi Heavy AC Back Online

Be Cool Refrigeration are F-Gas certified Mitsubishi Heavy Industries engineers covering London and the Home Counties. We carry MHI sensors, drain pumps, and common PCBs on the van, so most faults are diagnosed and repaired in a single visit. Whether your system needs a quick repair, a planned service, or a full replacement, we can have an engineer with you the same day on most callouts. Get in touch for a same-day Mitsubishi Heavy AC repair, or check our air conditioning installation and AC servicing options.