P6 on a Mitsubishi Electric heat pump: Freezing/Overheating protection operation
Short answer
On a Mitsubishi Electric heat pump, P6 means: Freezing/Overheating protection operation. 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 · 16 sources
What it means
On the Ecodan
The manuals for this range word it in different ways:
- Freezing/Overheating protection operation.
- Anti-freeze protection of plate heat exchanger.
Part involved: Plate heat exchanger / TH2 / water flow.
When it shows: Freeze or overheat protection operated at the FTC2 heat exchanger FTC4-FTC6: cooling - TH2 <= -5C for 10 s after 6 min compressor run; defrost - THW2 <= 15C and TH2 <= -16C for 10 s. FTC7: refrigerant temperature drops rapidly in cooling/defrost (TH2 or pressure sensor; depends on plate HEX capacity code).
Likely causes:
- Reduced water flow; abnormal water temperature; refrigerant circuit condition.
- 1 Reduced water flow (clogged filter, water leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control (cooling); 5 Overcharge / leakage or shortage of refrigerant; 6 Defective refrigerant circuit (clogs); 7 Malfunction of linear expansion valve; 8 Malfunction of pressure sensor / incorrect capacity code (FTC7).
On the Ecodan Hydrodan
The manual says: Anti-freeze protection of plate heat exchanger.
Part involved: Plate heat exchanger / TH2 / water flow.
When it shows: FTC4-FTC6: cooling - TH2 <= -5C for 10 s after 6 min compressor run; defrost - THW2 <= 15C and TH2 <= -16C for 10 s. FTC7: refrigerant temperature drops rapidly in cooling/defrost (TH2 or pressure sensor; depends on plate HEX capacity code).
Likely causes:
- 1 Reduced water flow (clogged filter, water leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control (cooling); 5 Overcharge / leakage or shortage of refrigerant; 6 Defective refrigerant circuit (clogs); 7 Malfunction of linear expansion valve; 8 Malfunction of pressure sensor / incorrect capacity code (FTC7).
On the Ecodan Puhz Sw Split
The manual says: Freezing/overheating protection is working - overheating protection in heating mode (split outdoor unit).
Part involved: Pressure sensor 63HS / refrigerant charge / water flow at indoor plate HEX.
When it shows: Heating: condensing temperature from 63HS >= Tcond (ambient-dependent, approx. 63C at -18C falling to 50-51C at higher ambients) with compressor frequency <= 25 Hz; not detected during defrost.
Likely causes:
- 1 Overcharge of refrigerant; 2 Defective refrigerant circuit (clogs); 3 Malfunction of linear expansion valve; 4 Reduced water flow (clogged filter, water leak); 5 High temperature (overload, inlet water too warm); 6 Defective water pump.
On the Ecodan Puhz W Monobloc
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection detected by outdoor unit).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Cooling: plate HEX pipe TH6 <= -5C for 10 s, or <= -2C at minimum frequency for 5 min after 6 min running. Heating/defrost (inlet water TH32 <= 15C): TH6 <= -6C for 30 s after 1 min of defrost, or TH6 <= -16C for 10 s during defrost.
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Overcharge of refrigerant; 6 Clogged refrigerant circuit; 7 LEV malfunction. Heating: 1 Reduced water flow; 2 Low temperature; 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 LEV malfunction.
On the Ecodan R290 Puz Wz
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection detected by outdoor unit).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Refrigerant saturation temperature (WZ50-80 also plate HEX liquid TH34) below a dynamically calculated threshold for 10 s; OCH880 describes freezing protection (cooling) and overheating protection (heating).
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, water leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Leakage or shortage of refrigerant; 6 Defective refrigerant circuit (clogs); 7 Malfunction of LEV. Heating/defrost: 1 Reduced water flow; 2 Low temperature (low load, cold inlet water); 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 Malfunction of LEV.
On the Ecodan R32 Puz Wm
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection detected by outdoor unit).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Plate HEX liquid temperature (TH34) or refrigerant saturation temperature below a dynamically calculated threshold for 10 s.
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, water leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Leakage or shortage of refrigerant; 6 Defective refrigerant circuit (clogs); 7 Malfunction of LEV. Heating/defrost: 1 Reduced water flow; 2 Low temperature (low load, cold inlet water); 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 Malfunction of LEV.
On the Ecodan R32 Zubadan Puz Hwm
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection detected by outdoor unit).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Plate HEX liquid temperature (TH34) or refrigerant saturation temperature below a dynamically calculated threshold for 10 s.
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, water leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Leakage or shortage of refrigerant; 6 Defective refrigerant circuit (clogs); 7 Malfunction of LEV. Heating/defrost: 1 Reduced water flow; 2 Low temperature (low load, cold inlet water); 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 Malfunction of LEV.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Cooling: plate HEX pipe TH6 <= -5C for 10 s, or <= -2C at minimum frequency for 5 min after 6 min running. Heating/defrost (TH34 inlet water <= 15C): TH6 <= -6C for 30 s after 1 min of defrost, or TH6 <= -16C for 10 s during defrost.
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Overcharge of refrigerant; 6 Clogged refrigerant circuit; 7 LEV malfunction. Heating: 1 Reduced water flow; 2 Low temperature; 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 LEV malfunction.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Freezing/overheating protection is working (plate heat exchanger freeze protection detected by outdoor unit).
Part involved: Plate heat exchanger / water flow / refrigerant charge.
When it shows: Cooling: plate HEX pipe TH6 <= -5C for 10 s, or <= -2C at minimum frequency for 5 min after 6 min running. Heating/defrost (inlet water TH32 <= 15C): TH6 <= -6C for 30 s after 1 min of defrost, or TH6 <= -16C for 10 s during defrost.
Likely causes:
- Cooling: 1 Reduced water flow (clogged filter, leak); 2 Low temperature (low load, inlet water too cold); 3 Defective water pump; 4 Defective outdoor fan control; 5 Overcharge of refrigerant; 6 Clogged refrigerant circuit; 7 LEV malfunction. Heating: 1 Reduced water flow; 2 Low temperature; 3 Defective water pump; 4 Leakage or shortage of refrigerant; 5 LEV malfunction.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Freezing/overheating protection is working - overheating protection in heating mode (split outdoor unit).
Part involved: Pressure sensor 63HS / refrigerant charge / water flow at indoor plate HEX.
When it shows: Heating: condensing temperature from 63HS >= Tcond (ambient-dependent, approx. 63C at -18C falling to 50-51C at higher ambients) with compressor frequency <= 25 Hz; not detected during defrost.
Likely causes:
- 1 Overcharge of refrigerant; 2 Defective refrigerant circuit (clogs); 3 Malfunction of linear expansion valve; 4 Reduced water flow (clogged filter, water leak); 5 High temperature (overload, inlet water too warm); 6 Defective water pump.
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 Hydrodan
| 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 Puhz Sw Split
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan Puhz W Monobloc
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan R290 Puz Wz
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan R32 Puz Wm
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan R32 Zubadan Puz Hwm
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan Ultra Quiet Puhz W Vaa
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan Zubadan Puhz Hw Monobloc
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
On the Ecodan Zubadan Puhz Shw Split
| 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 in the service manual; code is logged in the outdoor unit error history.
Manual reset: Clear via RESET on the main remote controller after the cause is fixed; service manual actions include switching power off and on to recheck.
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
- Check water flow, strainer, pump; check refrigerant charge.
- 1-2 Check water piping, filter, leaks; 3 Check water pump; 4 Check outdoor fan motor; 5-6 Check refrigerant circuit operating condition, correct refrigerant amount; 7 Check LEV; 8 Check pressure sensor / DIP SW5-3 to 5-7 capacity code. Install manuals: 'See Action for L3. Check for correct amount of refrigerant.'.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
On the Ecodan Hydrodan
- 1-2 Check water piping, filter, leaks; 3 Check water pump; 4 Check outdoor fan motor; 5-6 Check refrigerant circuit operating condition, correct refrigerant amount; 7 Check LEV; 8 Check pressure sensor / DIP SW5-3 to 5-7 capacity code. Install manuals: 'See Action for L3. Check for correct amount of refrigerant.'.
Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.
On the Ecodan Puhz Sw Split
- 1-2 Check operating condition of refrigerant circuit; 3 Check linear expansion valve; 4-5 Check water piping; 6 Check water pump.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Puhz W Monobloc
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R410A) (if refrigerant circuit work is needed).
On the Ecodan R290 Puz Wz
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification plus competence for flammable A3 refrigerant (R290) (if refrigerant circuit work is needed).
On the Ecodan R32 Puz Wm
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable) (if refrigerant circuit work is needed).
On the Ecodan R32 Zubadan Puz Hwm
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable) (if refrigerant circuit work is needed).
On the Ecodan Ultra Quiet Puhz W Vaa
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R410A) (if refrigerant circuit work is needed).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1-2 Check water piping (filter/strainer, leaks, flow); 3 Check water pump; 4 Check outdoor fan motor (cooling); 5-7 Check refrigerant circuit operating condition / correct refrigerant amount; check linear expansion valve.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas) / heating engineer (water circuit).
Certification: UK F-gas Category I certification (R410A) (if refrigerant circuit work is needed).
On the Ecodan Zubadan Puhz Shw Split
- 1-2 Check operating condition of refrigerant circuit; 3 Check linear expansion valve; 4-5 Check water piping; 6 Check water pump.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
What you might notice
What owners and engineers report
27 owner and engineer reports mention this.
- No heating on the Ecodan: 9 reports
- No hot water on the Ecodan: 9 reports
- Frequent defrosting on the Ecodan: 3 reports
- Pressure keeps dropping on the Ecodan: 2 reports
- Frequent defrosting on the Ecodan Puhz W Monobloc: 1 report
- Pressure keeps dropping on the Ecodan Puhz W Monobloc: 1 report
- Frequent defrosting on the Ecodan R32 Puz Wm: 1 report
- Pressure keeps dropping on the Ecodan R32 Puz Wm: 1 report
Sources
- Ecodan PAC-IF031B-E Installation Manual ↗ · PAC-IF031B-E installation manual · page 21 (7-2 Self-check) · Reviewed 23 September 2026
- Ecodan FTC2B - PAC-IF032B-E Installation Manual & Instruction Book (BH79D360H04) ↗ · BH79D360H04 · page 22 (6.2 Self-check) · Reviewed 23 September 2026
- Ecodan FTC2BR PAC-IF033B-E Installation Manual (BH79D852H01) ↗ · BH79D852H01 · page EN-22 (6.2 Self-check) · Reviewed 23 September 2026
- Ecodan EHSC-VM2B / EHPX-VM2B Service Manual ↗ · OCH532A · page 55-56 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan FTC5 - EHPT20X-MHCW, EHST20(D)(C)-MHCW Cylinder Service Manual (OCH570C) ↗ · OCH570C · page 66 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MHEDWR1 Service Manual (OCH714E) ↗ · OCH714E · page 58 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 63 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan Hydrodan EHWT17D-MHEDW Installation Manual RK79D203K04 ↗ · RK79D203K04 · page 48 (PDF p.49, Error Codes (FTC)) · Reviewed 23 September 2026
- Ecodan PUHZ-SW(75-100-120)(V-Y)HA-BS Service Manual (OCH755) ↗ · OCH755 · page OCH755 p.29; OCH525C p.28 · Reviewed 23 September 2026
- Ecodan PUHZ-W112VHA-BS Service Manual (OCH562D) ↗ · OCH562D · page OCH562D p.24; OCH605A p.21; OCH465D p.26; OCH439N p.61 · Reviewed 23 September 2026
- PUZ-WZ(85-100-120)(V-Y)AA(-BS) Service_Manual (OCH880) ↗ · OCH880 · page OCH880 p.44-45; OCH816 p.41 · Reviewed 23 September 2026
- Ecodan PUZ-WM(50-60-85-112)V(H-A)A Service Manual (OCH727A) ↗ · OCH727A · page 31 · Reviewed 23 September 2026
- Ecodan PUZ-HWM140(V-Y)HA Service Manual (OCH748A) ↗ · OCH748A · page 27 · Reviewed 23 September 2026
- PUHZ-W85-112VAA(-BS) Service Manual (OCH681) ↗ · OCH681 · page 22 · Reviewed 23 September 2026
- Ecodan PUHZ-HW140VHA2-BS / PUHZ-HW140YHA2-BS Service Manual (OCH439N) ↗ · OCH439N · page 61 · Reviewed 23 September 2026
- PUHZ-SHW112-140V/YHA Service Manual (OCH526) ↗ · OCH526 · page 32 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026