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Heat pumps · Error codes · Mitsubishi Electric
L5Mitsubishi Electric

L5 on a Mitsubishi Electric heat pump: Thermistor (Return water temp.) (THW2) failure

Short answer

On a Mitsubishi Electric heat pump, L5 means: Thermistor (Return water temp.) (THW2) failure. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.

Reviewed 23 September 2026 · 8 sources

What it means

On the Ecodan

The manuals for this range word it in different ways:

  • Thermistor (Return water temp.) (THW2) failure.
  • Indoor unit temperature thermistor (THW1, THW2, THW5, THW6, THW7, THW8, THW9) failure.
  • Indoor unit temperature thermistor (THW1, THW2, THW5B, THW6, THW7, THW8, THW9) failure.
  • Indoor unit temperature thermistor (THW1, THW2, THW5A, THW5B, THW6, THW7, THW8, THW9) failure.

Part involved: Return water thermistor THW2; FTC water thermistors THW1-THW9; FTC water thermistors THW1, THW2, THW5B, THW6-THW9; FTC water thermistors THW1, THW2, THW5A, THW5B, THW6-THW9.

When it shows: Open/short of THW2 (LED4 blink, LED5 off) Thermistor open (-39C or below) or short (88.5C or above); failed thermistor identified in Running information request code 567.

Likely causes:

  • Thermistor THW2 open/short or disconnected.
  • 1 Connector/terminal wire detached or loose; 2 Thermistor fault; 3 FTC board failure; 4 Incorrect DIP switch setting.

On the Ecodan Co2 Quhz

The manual says: Indoor unit temperature thermistor (THW1, THW2, THW3, THW4, THW5A, THW5B) failure.

Part involved: Thermal store thermistors THW1-THW5B.

When it shows: Thermistor open or short (request code 567 identifies it).

Likely causes:

  • Connector/terminal detached; thermistor fault; FTC board failure.

On the Ecodan Hydrodan

The manual says: FTC temperature thermistor (THW1, THW2, THW5A, THW5B, THW6, THW7, THW8, THW9) failure.

Part involved: FTC water thermistors.

When it shows: Thermistor open (-39C or below) or short (88.5C or above); failed thermistor identified in Running information request code 567.

Likely causes:

  • 1 Connector/terminal wire detached or loose; 2 Thermistor fault; 3 FTC board failure; 4 Incorrect DIP switch setting.

Is it safe?

On the Ecodan

Does the heat pump stop?Partly
SeverityFault — needs a manual reset once the cause is fixed
Engineer needed?Yes

The heat pump keeps running with some functions stopped.

Resetting itself: Not stated as auto-resetting; code remains until reset.

Manual reset: Installation manuals: press F4 (RESET) then F3 (Yes) on the main remote controller (older controllers: hold button E for 3 s); 'To cancel error codes please switch system off'. Fix the cause first.

On the Ecodan Co2 Quhz

Does the heat pump stop?Partly
SeverityFault — needs a manual reset once the cause is fixed
Engineer needed?Yes

The heat pump keeps running with some functions stopped.

Resetting itself: Not stated as auto-recovering.

Manual reset: Press F4 (Reset) then F3 (Yes) on the main remote controller, or switch system off (installation manual).

On the Ecodan Hydrodan

Does the heat pump stop?Partly
SeverityFault — needs a manual reset once the cause is fixed
Engineer needed?Yes

The heat pump keeps running with some functions stopped.

Resetting itself: Not stated as auto-resetting; code remains until reset.

Manual reset: Installation manuals: press F4 (RESET) then F3 (Yes) on the main remote controller (older controllers: hold button E for 3 s); 'To cancel error codes please switch system off'. Fix the cause first.

What you can check

  • Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing.

What the engineer will check

On the Ecodan

  • Check resistance across the thermistor.
  • 1 Visually check terminals/connections; 2 Check thermistor resistance against table, compare with hand-held thermometer; 3 Replace FTC board; 4 Check DIP switch settings.

Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).

On the Ecodan Co2 Quhz

  • Check connections; check resistance against table; replace board.

Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).

On the Ecodan Hydrodan

  • 1 Visually check terminals/connections; 2 Check thermistor resistance against table, compare with hand-held thermometer; 3 Replace FTC board; 4 Check DIP switch settings.

Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).

What you might notice

What owners and engineers report

19 owner and engineer reports mention this.

  • No heating on the Ecodan: 9 reports
  • No hot water on the Ecodan: 9 reports
  • Cylinder or buffer thermostat/sensor problem on the Ecodan: 1 report

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026