Mitsubishi Ecodan Error Codes
Basic homeowner checks only. Never remove covers or work on electrics/refrigerant – contact a qualified F-Gas engineer or your installer for anything beyond visual checks and controller resets.
Critical – risk of damage / immediate shutdown High – system likely off, needs attention Medium – limited operation / nuisance fault Low – minor / advisory
Description
Low-pressure safety has operated. Often linked to refrigerant shortage, closed service valves or restrictions on the refrigerant side.
Checks (Homeowner)
  • Look for obvious oil stains on external pipework that could indicate a leak.
  • Note any unusual noises from the outdoor unit.
Actions
  • Switch the system off and do not repeatedly reset.
  • Contact an F-Gas engineer – refrigerant checks must be done professionally.
Description
High-pressure safety has activated or its circuit is faulty. Often linked to poor water flow, blocked strainers or restricted airflow.
Checks (Homeowner)
  • Check all isolation valves on the heating side are open.
  • Check strainers/filters if accessible (near indoor unit) and note if dirty.
  • Ensure outdoor unit has clear airflow (no leaves, covers, obstructions).
Actions
  • After obvious issues are cleared, you may reset once.
  • If F5 returns, stop resetting and call an engineer – do not bypass safeties.
Description
The unit is seeing an impossible combination of high and low pressure switch states, usually due to wiring or board faults, occasionally severe system issues.
Checks (Homeowner)
  • No safe internal checks – just note if this appeared during heating or DHW.
Actions
  • Turn the system off and contact your installer/F-Gas engineer.
  • They will check pressure switch wiring, continuity and the PCB.
Description
The controller has detected incorrect wiring between the FTC and the heat pump, commonly on S1/S2/S3 or similar control terminals.
Checks (Homeowner)
  • Think back to any recent work: new controls, PV diverter, Wi-Fi stat, etc.
  • Do not open covers or touch internal wiring.
Actions
  • Contact the original installer or a competent heat pump electrician.
  • Advise them of EA/EB and any recent wiring changes.
Description
The heat pump fails its start-up checks. Could be due to sensor faults, communication issues, incorrect dip-switches or wiring.
Checks (Homeowner)
  • Note whether this happens on heating, DHW or both.
  • Confirm mains supply is not tripping and no other error appears first.
Actions
  • Record the code and conditions and contact your installer.
  • Commissioning checks (dip-switches, addresses, wiring) are installer-only.
Description
A communication transmit error has been detected between FTC and other boards or controllers.
Checks (Homeowner)
  • Check for obvious damage to visible control cables (chewed, pinched).
  • If the controller is remote wired, ensure it hasn’t been knocked or flooded.
Actions
  • Try a power cycle (off at isolator for 5 minutes, then back on).
  • If the error reappears, contact your installer – they’ll check comms wiring and boards.
Description
The main or remote controller is reporting an internal fault or cannot operate correctly.
Checks (Homeowner)
  • Check for visible water ingress or condensation on the controller.
  • Confirm that any recently fitted controls are correctly wired (by installer).
Actions
  • Do not continue if you smell burning or see scorching – isolate power and call an engineer.
  • In most cases the controller or wiring will need professional diagnosis/replacement.
Description
Communication from another board/controller is not being received correctly.
Checks (Homeowner)
  • Check that any external controls (stats, timers) are powered and functioning.
  • Check isolators/fused spurs feeding the cylinder/hydrobox are switched on.
Actions
  • Power cycle once. If E4/E5 returns, an installer must check control wiring & boards.
Description
Communication between indoor (FTC/cylinder/hydrobox) and outdoor unit has failed or is unstable.
Checks (Homeowner)
  • Check all relevant isolators are on (outdoor unit and indoor unit supplies).
  • Look for visible cable damage between indoor and outdoor (chewed, crushed).
Actions
  • Try one power reset to both indoor and outdoor supplies.
  • If the fault comes back, an engineer must test the comms cable and boards.
Description
A miscellaneous error the controller cannot categorise, often due to electrical noise or a transient glitch.
Checks (Homeowner)
  • Note if anything else has recently been installed (EV charger, PV inverter, etc.).
  • Check for repeated nuisance tripping of other circuits in the house.
Actions
  • Reset once. If EF appears frequently, ask an electrician/installer to check supply quality and earthing.
Description
The outdoor main board and inverter board are not communicating properly.
Checks (Homeowner)
  • No safe internal checks – boards and connectors are behind covers.
Actions
  • Record the fault and conditions, then contact an F-Gas/approved Mitsubishi engineer.
Description
Similar to F5: discharge pressure is too high. Very often caused by poor water flow or blocked heat exchanger / strainers.
Checks (Homeowner)
  • Ensure radiators/UFH zones are open and calling for heat.
  • Check for blocked strainers/filters if they have homeowner-accessible isolation valves.
  • Clear any debris/obstructions around the outdoor coil.
Actions
  • Reset once after obvious issues are cleared.
  • If U1 returns, stop using the system and call an engineer – ongoing high-pressure operation risks damage.
Description
The compressor is running too hot. Causes include low airflow, incorrect refrigerant charge or heavy load at low ambient.
Checks (Homeowner)
  • Check outdoor coil for dirt, leaves, snow or ice; keep clear around intake/exhaust.
  • Note any abnormal noise or vibration from the outdoor unit.
Actions
  • Do not attempt any refrigerant work.
  • Log conditions (outside temp, mode, run time) and call an F-Gas engineer.
Description
The discharge temperature sensor/thermistor is open, shorted or outside its expected range.
Checks (Homeowner)
  • No internal checks – sensor is in the outdoor unit.
Actions
  • Contact your installer; they will test/replace the sensor or wiring.
Description
One or more temperature sensors are disconnected or shorted.
Checks (Homeowner)
  • If you can see external sensor leads (e.g. outdoor air) check they’re not physically damaged.
Actions
  • Report to your installer – they will test resistance and replace the faulty thermistor.
Description
The inverter electronics are overheating – possibly due to blocked airflow, high load or internal faults.
Checks (Homeowner)
  • Ensure outdoor unit airflow is not obstructed.
Actions
  • If U5 recurs, stop using the system and call an engineer – continued use risks PCB damage.
Description
The compressor is drawing excessive current, often due to mechanical or electrical faults.
Checks (Homeowner)
  • No user checks – this is an electrical/refrigerant issue.
Actions
  • Turn off at the isolator and call an F-Gas / refrigeration engineer.
Description
The outdoor fan is not starting or its feedback is abnormal.
Checks (Homeowner)
  • Look (safely) to see if the fan is spinning when the unit is running.
  • Check for obstructions (ice, debris) preventing fan rotation.
Actions
  • Do not poke the fan blades; remove loose debris only if clearly safe.
  • If U8 persists, an engineer must test/replace the fan motor or PCB.
Description
The supply voltage is outside the acceptable band.
Checks (Homeowner)
  • See if other appliances are also misbehaving or lights dimming/flickering.
Actions
  • Contact your electrician or DNO if you suspect a supply issue.
  • If U9 is intermittent, an electrician should check connections, fuses and earth.
Description
A component has exceeded its safe temperature limit (often electrical parts).
Checks (Homeowner)
  • If you smell burning or see scorch marks, isolate power immediately.
Actions
  • Do not reset and continue use; call an engineer urgently.
Description
The compressor is drawing excessive current or is mechanically locked.
Checks (Homeowner)
  • No safe DIY checks – this is an internal compressor/inverter fault.
Actions
  • Switch the unit off and call an F-Gas engineer.
Description
The unit detects current outside normal operating range – can be supply, compressor or inverter related.
Checks (Homeowner)
  • Note if other devices trip when the heat pump starts.
Actions
  • Have an electrician/engineer check supply, wiring and the outdoor unit.
Description
A monitored circuit is open when it should be closed – usually broken wiring or a failed component.
Checks (Homeowner)
  • Check visible cables for obvious physical damage only.
Actions
  • Report UL to your installer; they’ll trace and repair the open circuit.
Description
Similar to U6/UH – excessive current detected in a protected circuit.
Checks (Homeowner)
  • No user checks; this is a job for an electrician/engineer.
Actions
  • Isolate the unit and arrange professional diagnosis.
Description
The main flow temperature sensor (thermistor) is reading incorrectly, open or shorted.
Checks (Homeowner)
  • Check if the controller is showing unrealistic flow temperatures (e.g. –50°C).
Actions
  • Sensors are inexpensive; ask your installer to test/replace the P1 thermistor.
Description
Water side of the plate heat exchanger is too hot, usually due to low water flow or air in the system.
Checks (Homeowner)
  • Check system pressure on the gauge (installer will have given a normal range).
  • Bleed accessible radiators if you’re comfortable and they’re gurgling/airbound.
Actions
  • If P6 returns, the system needs a proper flow/air/strainer check by an engineer.
Description
The controller isn’t seeing a normal temperature difference across the plate heat exchanger – could be a pump, flow or refrigerant-side issue.
Checks (Homeowner)
  • Make sure there is a genuine demand (heating/DHW calling) when P8 appears.
  • Listen to the circulating pump – note if unusually noisy or silent.
Actions
  • Reset once; if the code returns, call your installer to check pump performance and refrigerant circuit.
Description
A specific sensor (TH5 – model dependent) is reporting invalid values or is disconnected.
Checks (Homeowner)
  • No user intervention required apart from noting when it occurs.
Actions
  • Provide the code and model number to your engineer so they can locate and check TH5.
Description
The inlet/return water temperature sensor is out of range or behaving unexpectedly.
Checks (Homeowner)
  • Check that heating/DHW has been running long enough for stable readings.
  • Note any obviously incorrect temperature displayed for return/inlet water.
Actions
  • Ask your installer to test and, if needed, replace the inlet thermistor or wiring.
No error codes match your search. Try a different code (e.g. U1, P6) or keyword.