L3 on a Mitsubishi Electric heat pump: Circulation water temperature overheat protection (primary flow/return water too hot)
Short answer
On a Mitsubishi Electric heat pump, L3 means: Circulation water temperature overheat protection (primary flow/return water too hot). 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: Circulation water temperature overheat protection (primary flow/return water too hot).
Part involved: Flow water thermistor THW1 / return THW2 / primary circuit flow.
When it shows: THW1 or THW2 >= 80C for 10 consecutive seconds (FTC4/FTC5/FTC6 service manuals); >= 85C (FTC7 OCH814A).
Likely causes:
- 1 Insufficient system head (pump undersized); 2 Reduced flow in primary water circuit (faulty pump, insufficient air purge, blocked strainer/magnetic filter, leak in water circuit); 3 Valve operation fault (valves not installed level); 4 2-way valve (local supply) actuator fault; 5 3-way valve actuator fault; 6 Booster heater relay (BHC1/BHC2/BHCP) operating fault; 7 Power supply voltage increase; 8 THW1 or THW5(B) detached from holder; 9 THW1/THW2 fault; 10 FTC board failure.
On the Ecodan Co2 Quhz
The manual says: Circulation water temperature overheat protection.
Part involved: THW1/THW2/THW3 flow thermistors.
When it shows: THW1, THW2 or THW3 above limit for consecutive seconds (numeric thresholds not legible in the text layer).
Likely causes:
- Reduced flow in primary water circuit (faulty pump, insufficient air purge, blocked strainer, leak); valve operation fault; 2-way (local) / 3-way valve actuator fault; booster heater relay (CNBH) fault; supply voltage increase; THW1/THW2 detached; THW1/THW2/THW3 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. Check the primary circuit pressure gauge (typically about 1-2 bar when cold on sealed systems) and look for visible leaks; do not top up or bleed unless your installer has shown you how and the filling loop is then closed.
What the engineer will check
On the Ecodan
- 1 Check pump against required head, add/replace pump if needed; 2 Check circulation pump, open purge valve to remove trapped air, check strainer for blockage, check primary circuit for leaks, check flow rate within recommended range; 3 Check valves installed level; 4-5 Electrically test valve actuators, operate 3-way valve manually from service menu, replace coil/valve; 6 Electrically test booster heater relays; 7 Check supply voltage; 8 Reattach thermistors; 9 Check thermistor resistance / compare with hand-held thermometer; 10 Replace FTC board.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
On the Ecodan Co2 Quhz
- Check circulation pump, bleed air, check strainer/leaks/flow; check valves, 3-way valve, CNBH relays, supply voltage, thermistors; replace board.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit).
What you might notice
What owners and engineers report
25 owner and engineer reports mention this.
- No heating on the Ecodan: 9 reports
- No hot water on the Ecodan: 9 reports
- Keeps stopping / cutting out on the Ecodan: 6 reports
- Radiators cold at top / air in system 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