What it means
Mitsubishi's title is "Circulation water freeze protection". The service manual says it shows when sensor THW1 reads 1°C or less, or THW2 reads 3°C or less, for 10 seconds in a row. On systems that also cool, Mitsubishi's FTC6 controller manual says L6 can appear during hot water or legionella runs that follow cooling, and the fix is an installer setting (the freeze stat temperature).
What to check, in order
- Keep the power on
Mitsubishi's FTC7 operation manual warns that freeze protection does not work while the unit is off or unpowered. Don't switch it off at the mains in cold weather.
- Write the code down and reset once
Switch the system off at the controller, clear the code, and switch it back on.
- Look for leaks
Mitsubishi's action for L6 is the same as for L3: check for water leakage, a blocked strainer and the circulation pump. Look for drips and damp patches.
When to call your installer
If L6 comes back, call your installer. Mitsubishi lists low flow (a weak or faulty pump, trapped air, a blocked strainer, a leak), 2-way or 3-way valve faults, a temperature sensor that has come loose or failed, and a control board fault.
Quick answers
What does the Mitsubishi Ecodan L6 error code mean?
The water in the heating circuit has got close to freezing, so the system has stepped in to protect itself. Mitsubishi's title is "Circulation water freeze protection". The service manual says it shows when sensor THW1 reads 1°C or less, or THW2 reads 3°C or less, for 10 seconds in a row. On systems that also cool, Mitsubishi's FTC6 controller manual says L6 can appear during hot water or legionella runs that follow cooling, and the fix is an installer setting (the freeze stat temperature).
Can I fix Mitsubishi Ecodan L6 myself?
Moderate (about 15 minutes). Start with: keep the power on; write the code down and reset once; look for leaks. If L6 comes back, call your installer. Mitsubishi lists low flow (a weak or faulty pump, trapped air, a blocked strainer, a leak), 2-way or 3-way valve faults, a temperature sensor that has come loose or failed, and a control board fault.
Is Mitsubishi Ecodan L6 worth repairing?
Worth fixing. Worth sorting quickly in cold weather. Mitsubishi points first to low water flow, which an installer can normally trace.
Checked against
- Ecodan Hydrobox EHSC/EHSD series (D models) Installation Manual, error code table (manualslib copy)
- Ecodan Hydrobox EHSD/ERSD series (E models) Installation Manual DG79T728H01, error code table (manualslib copy)
- Ecodan Cylinder Unit Service Manual OCH714A, EHST/ERST/EHPT D models (PDF)
- Ecodan FTC6 PAC-IF07(1-3)B-E Installation Manual (BH79D843H06) (PDF)
- Ecodan EHPT(15-17-21-25-30)X-UKH(L)EW1(S-L) Operation Manual, FTC7 (758753) (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.