P8 on a Mitsubishi Electric heat pump: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start)
Short answer
On a Mitsubishi Electric heat pump, P8 means: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start). 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 · 7 sources
What it means
On the Ecodan Puhz W Monobloc
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: 3 min after 10 min compressor run at >= 61 Hz: heating T63HS-TH32 <= 2C and TH6-TH32 <= 1C and TH7-TH3 <= 1C (cooling equivalents).
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan R290 Puz Wz
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: Conditions continue 3 minutes after 10 minutes of compressor operation - heating: T63HS-THW2 <= 2C and TH6-THW2 <= 1C and TH7-TH3 <= 1C (cooling equivalents).
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan R32 Puz Wm
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: 3 min continuously after 10 min compressor run: heating T63HS-THW2 <= 2C and TH6-THW2 <= 1C and TH7-TH3 <= 1C and comp. >= 61 Hz; cooling TH6-TH7 <= 2C and TH3-TH7 <= 4C (or TH6-TH3 < 0C) and THW2-TH34 <= 0C and >= 61 Hz.
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan R32 Zubadan Puz Hwm
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: 3 min continuously after 10 min compressor run: heating T63HS-THW2 <= 2C and TH6-THW2 <= 1C and TH7-TH3 <= 1C (cooling equivalents).
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: 3 min after 10 min compressor run: heating T63HS-TH34 <= 2C and TH6-TH34 <= 1C and TH7-TH3 <= 1C and >= 61 Hz (cooling equivalents).
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Pipe temperature abnormality (refrigerant temperatures do not change as expected after compressor start).
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: 3 min after 10 min compressor run at >= 61 Hz: heating T63HS-TH32 <= 2C and TH6-TH32 <= 1C and TH7-TH3 <= 1C (cooling equivalents).
Likely causes:
- 1 Leakage or shortage of refrigerant; 2 Malfunction of linear expansion valve; 3 Refrigerant circuit clogged with foreign objects (water ingress freezing); 4 Disconnection of thermistor holder.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Pipe temperature abnormality (split): indoor pipe (liquid/condenser) temperature not within expected heating/cooling range after compressor start.
Part involved: Refrigerant circuit thermistors / LEV.
When it shows: Cooling: pipe temperature not in cooling range 3 min after start and 6 min after leaving range (>= 9 min to detect; not in drying mode); heating: condenser/evaporator pipe temperature not in heating range within 20 min after hot adjustment.
Likely causes:
- 1 Slight temperature difference between room and pipe thermistors (refrigerant shortage, disconnected thermistor holder, defective refrigerant circuit); 2 Converse connection of extension pipe (plural units); 3 Converse indoor/outdoor wiring (plural units); 4 Defective room/pipe thermistor detection; 5 Stop valve not fully opened.
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-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.
What the engineer will check
On the Ecodan Puhz W Monobloc
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan R290 Puz Wz
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification plus competence for flammable A3 refrigerant (R290).
On the Ecodan R32 Puz Wm
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable).
On the Ecodan R32 Zubadan Puz Hwm
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R32, A2L mildly flammable).
On the Ecodan Ultra Quiet Puhz W Vaa
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1 Check intake superheat and refrigerant leakage; 2 Check linear expansion valve; 3 Recover refrigerant and evacuate >1 hour; 4 Check temperature display (SW2) and thermistor holders.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (refrigeration / F-gas).
Certification: UK F-gas Category I certification (R410A).
On the Ecodan Zubadan Puhz Shw Split
- 1 Check thermistor holders and refrigerant charge; 2-3 Check pipe and wiring connections (plural units); 4 Check thermistor readings; 5 Open stop valves fully.
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
1 owner and engineer report mentions this.
- Insufficient heating (house not warm enough) on the Ecodan Zubadan Puhz Hw Monobloc: 1 report
Sources
- Ecodan PUHZ-W112VHA-BS Service Manual (OCH562D) ↗ · OCH562D · page OCH562D p.25; OCH605A p.22; 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.43-44; 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