L6 on a Mitsubishi Electric heat pump: Circulation water freeze protection
Short answer
On a Mitsubishi Electric heat pump, L6 means: Circulation water freeze protection. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
On the Ecodan
The manual says: Circulation water freeze protection.
Part involved: Flow/return thermistors THW1/THW2; primary circuit flow.
When it shows: THW1 <= 1C or THW2 <= 3C for 10 consecutive seconds (not displayed if freeze stat function disabled, or for 10 min after pump 1 starts).
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 THW1 detached from holder; 7 THW1 or THW2 fault; 8 FTC board failure.
On the Ecodan Co2 Quhz
The manual says: Circulation water freeze protection.
Part involved: THW1/THW2.
When it shows: THW1 or THW2 below limit (not displayed if freeze stat disabled or within 10 min of pump1 start).
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; THW1/THW2 detached or faulty; 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 Reattach thermistor; 7 Check thermistor resistance; 8 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
- As L3: check pump, air, strainer, leaks, valves, thermistors; 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 heating on the Ecodan: 9 reports
- Stops working in very cold weather on the Ecodan: 2 reports
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 60 (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