L4 on a Mitsubishi Electric heat pump: DHW tank water temperature overheat protection
Short answer
On a Mitsubishi Electric heat pump, L4 means: DHW tank water temperature overheat protection. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Ecodan
The manual says: DHW tank water temperature overheat protection.
Part involved: DHW tank thermistor THW5 / THW5B; immersion heater & contactor; 3-way valve.
When it shows: THW5 (FTC4/FTC5) / THW5B (FTC6/FTC7) >= 75C for 10 consecutive seconds.
Likely causes:
- 1 3-way valve actuator fault; 2 Immersion heater relay (IHC) operating fault (install manuals: check the immersion heater and its contactor); 3 DHW tank thermistor fault; 4 FTC board failure.
On the Ecodan Co2 Quhz
The manual says: Thermal store tank water temperature overheat protection.
Part involved: Stored water thermistor THW5A / 3-way valve.
When it shows: THW5A above limit for consecutive seconds.
Likely causes:
- 3-way valve actuator fault; THW5A fault; FTC board failure.
Is it safe?
On the Ecodan
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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).
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
- 1 Electrically test 3-way valve, manual operation from service menu, replace coil/valve; 2 Check immersion heater relay/contactor; 3 Check thermistor resistance; 4 Replace board.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
On the Ecodan Co2 Quhz
- Electrically test 3-way valve / manual operation / replace coil or valve; check THW5A (and THW1, THW3) resistance; replace board.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit).
What you might notice
What owners and engineers report
11 owner and engineer reports mention this.
- No hot water on the Ecodan: 9 reports
- Hot water too hot / scalding (incl. legionella cycle) on the Ecodan: 1 report
- Immersion heater running constantly on the Ecodan: 1 report
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 59 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20Q VM2EA Service Manual (GCH15010C) ↗ · GCH15010C · page 37 (10-4 Self diagnosis and action) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026