U5 on a Mitsubishi Electric heat pump: Temperature of heat sink (TH8 over-temperature on inverter heat sink)
Short answer
On a Mitsubishi Electric heat pump, U5 means: Temperature of heat sink (TH8 over-temperature on inverter heat sink). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 9 sources
What it means
On the Ecodan Hydrodan
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink detects 95C.
Likely causes:
- 1 Rise of ambient temperature (upper limit 35C); 2 Defective thermistor; 3 Defective input circuit of power circuit board.
On the Ecodan Puhz Sw Split
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 77C (SW75V), 94C (SW100V/SW120V), 84C (SW100Y; SW40/50).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan Puhz W Monobloc
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 77C (W50VHA/W50VHA2, W85VHA/W85VHA2); W112VHA uses an internal power-module thermistor (short detection 170C).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan R290 Puz Wz
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 78C (all PUZ-WZ sizes).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan R32 Puz Wm
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 77C (WM50V), 78C (WM60V/85V/112V), 85C (WM85Y/112Y).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan R32 Zubadan Puz Hwm
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 >= 76C (HWM140VHA) / 95C (HWM140YHA).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 78C (W85V, W112V) / 85C (W85Y, W112Y).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 >= 84C (HW140VHA), 95C (HW112YHA/HW140YHA, HW112/140YHA2 R1-R4), 89C (HW140VHA2 R1-R7), 91C (HW112/140YHA2 R5/R7).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Temperature of heat sink (TH8 over-temperature on inverter heat sink).
Part involved: Heat sink thermistor TH8 / inverter power module.
When it shows: TH8 heat sink >= 95C (SHW80V, SHW112V) / 84C (SHW112Y, SHW140Y).
Likely causes:
- 1 Outdoor fan motor locked; 2 Failure of outdoor fan motor; 3 Airflow path clogged; 4 Rise of ambient temperature; 5 Defective thermistor; 6 Defective input circuit of outdoor power circuit board; 7 Failure of outdoor fan drive circuit.
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
On the Ecodan Hydrodan
- 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.
On the Ecodan Puhz Sw Split
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan Puhz W Monobloc
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan R290 Puz Wz
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan R32 Puz Wm
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan R32 Zubadan Puz Hwm
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan Ultra Quiet Puhz W Vaa
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan Zubadan Puhz Hw Monobloc
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
On the Ecodan Zubadan Puhz Shw Split
- 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. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
What the engineer will check
On the Ecodan Hydrodan
- 1 Check for heat sources around unit; power off/on and check U5 within 30 min; 2 Check TH8; 3 Replace power circuit board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz Sw Split
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz W Monobloc
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R290 Puz Wz
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Puz Wm
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Zubadan Puz Hwm
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Ultra Quiet Puhz W Vaa
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Shw Split
- 1-2 Check outdoor fan; 3 Check airflow path for cooling; 4 Check for anything raising temperature around outdoor unit (ambient upper limit 46C); power off/on and check if U5 recurs within 30 min (if U4 appears follow U4); 5 Check TH8 resistance; 6 Replace outdoor power board; 7 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- Keeps stopping / cutting out on the Ecodan R32 Puz Wm: 1 report
Sources
- Ecodan Hydrodan EHWT17D-MHEDW Installation Manual RK79D203K04 ↗ · RK79D203K04 · page 50 (PDF p.51) · Reviewed 23 September 2026
- Ecodan PUHZ-SW(75-100-120)(V-Y)HA-BS Service Manual (OCH755) ↗ · OCH755 · page OCH755 p.25; OCH525C p.23 · Reviewed 23 September 2026
- Ecodan PUHZ-W112VHA-BS Service Manual (OCH562D) ↗ · OCH562D · page OCH562D p.20; OCH605A p.17; OCH465D p.22; OCH439N p.56 · Reviewed 23 September 2026
- PUZ-WZ(85-100-120)(V-Y)AA(-BS) Service_Manual (OCH880) ↗ · OCH880 · page OCH880 p.39; OCH816 p.35 · Reviewed 23 September 2026
- Ecodan PUZ-WM(50-60-85-112)V(H-A)A Service Manual (OCH727A) ↗ · OCH727A · page 26 · Reviewed 23 September 2026
- Ecodan PUZ-HWM140(V-Y)HA Service Manual (OCH748A) ↗ · OCH748A · page 23 · Reviewed 23 September 2026
- PUHZ-W85-112VAA(-BS) Service Manual (OCH681) ↗ · OCH681 · page 17 · Reviewed 23 September 2026
- Ecodan PUHZ-HW140VHA2-BS / PUHZ-HW140YHA2-BS Service Manual (OCH439N) ↗ · OCH439N · page 56 · Reviewed 23 September 2026
- PUHZ-SHW112-140V/YHA Service Manual (OCH526) ↗ · OCH526 · page 27 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026