LC on a Mitsubishi Electric Ecodan: Boiler circulation water temperature overheat protection (hybrid/boiler systems)
Short answer
On a Mitsubishi Electric heat pump, LC means: Boiler circulation water temperature overheat protection (hybrid/boiler systems). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Boiler circulation water temperature overheat protection (hybrid/boiler systems).
Part involved: Boiler flow thermistors THWB1/THWB2; boiler circuit.
When it shows: THWB1 (or THWB2) >= 80C for 10 consecutive seconds.
Likely causes:
- 1 Boiler set temperature too high; 2 Reduced flow in boiler heating circuit (water leakage, strainer blockage, pump function).
Is it safe?
| 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.
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
- 1 Check boiler set temperature against restriction (thermistor kit PAC-TH011HT-E / PAC-TH012HT(L)-E manual); 2 Check for leaks, strainer blockage and pump function.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
What you might notice
What owners and engineers report
9 owner and engineer reports mention this.
- No heating on the Ecodan: 9 reports
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 62 (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