L8 on a Mitsubishi Electric heat pump: Heating operation error (flow temperature does not rise as expected / thermistor dislodged)
Short answer
On a Mitsubishi Electric heat pump, L8 means: Heating operation error (flow temperature does not rise as expected / thermistor dislodged). 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: Heating operation error (flow temperature does not rise as expected / thermistor dislodged).
Part involved: Flow thermistor THW1 / booster heater / zone thermistors THW6-THW9.
When it shows: Request code 567 shows 3: a) no change on THW1 and THW5(B) (<1C in 20 min from start), b) no change on THW1 (<1C in 10 min from booster heater start), c) THW1-THW2 < -5C for 10 min; 'A': THW6/THW7 zone 1; 'C': THW8/THW9 zone 2.
Likely causes:
- 1 THW1 (or THW6/THW8) detached from holder; 2 Booster heater fault; 3 THW1/THW2/THW5 (or zone thermistor) fault; 4 FTC board failure.
On the Ecodan Co2 Quhz
The manual says: Heating operation error.
Part involved: THW1 / booster heater.
When it shows: No change on THW1 (<1C in 20 min from start, or 10 min from booster heater start) or THW1-THW2 < -5C for 10 min.
Likely causes:
- THW1 detached; booster heater fault; THW1/THW2 fault; FTC board failure.
Is it safe?
On the Ecodan
| 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 Reattach thermistor; 2 Electrically test booster heater; 3 Check thermistor resistance; 4 Replace board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Co2 Quhz
- Reattach THW1; test booster heater; check thermistors; replace board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
What owners and engineers report
22 owner and engineer reports mention this.
- Insufficient heating (house not warm enough) on the Ecodan: 10 reports
- No heating on the Ecodan: 9 reports
- Unexpected backup / auxiliary heater use on the Ecodan: 3 reports
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 61 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20Q VM2EA Service Manual (GCH15010C) ↗ · GCH15010C · page 38 (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