U4 on a Mitsubishi Electric heat pump: Short circuit or disconnection of defrost temperature thermistor (TH2)
Short answer
On a Mitsubishi Electric heat pump, U4 means: Short circuit or disconnection of defrost temperature thermistor (TH2). 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 · 10 sources
What it means
On the Ecodan Co2 Quhz
The manuals for this range word it in different ways:
- Short circuit or disconnection of defrost temperature thermistor (TH2).
- Short circuit or disconnection of ambient temperature thermistor (TH3).
- Short circuit or disconnection of hot water temperature thermistor (TH4).
- Disconnection of outdoor inverter PCB temperature thermistor.
Part involved: Defrost thermistor TH2 (CN641); Ambient thermistor TH3 (CN643); Hot water thermistor TH4 (CN664); Board temperature thermistor (on inverter PCB).
When it shows: Disconnection: -47C or less for 2 min at compressor start; short: 153C or more for 2 min Disconnection: -44C or less for 2 min at compressor start; short circuit continued 2 min Disconnection: -5C or less for 2 min from 10 min after compressor start; short circuit continued 2 min Disconnection continued 2 min from 1 min after compressor start, or short circuit for 2 min.
Likely causes:
- 1 Defective connector CN641 connection; 2 Thermistor disconnection or short circuit; 3 Defective outdoor inverter PCB.
- 1 Defective connector CN643 connection; 2 Thermistor disconnection or short circuit; 3 Defective outdoor inverter PCB.
- 1 Defective connector CN664 connection; 2 Thermistor disconnection or short circuit; 3 Defective outdoor inverter PCB.
- 1 Defective board temperature thermistor.
On the Ecodan Hydrodan
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Heat pump unit thermistors TH3, TH32, TH34, TH7, TH8.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (C.B.: TH3, TH32, TH34, TH7; power board: CN6); 2 Defective thermistor; 3 Defective C.B.
On the Ecodan Puhz Sw Split
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH6/TH7/TH8/TH34.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Puhz W Monobloc
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3 (liquid), TH32 (inlet water), TH33/TH34, TH6 (plate HEX liquid), TH7 (ambient), TH8 (heat sink).
When it shows: Open or short detected during compressor operation (open detection of TH3/TH32/TH6 inoperative 10 s-10 min after start and during/10 min after defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan R290 Puz Wz
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH6/TH7/TH8/TH34.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH7/6 [+TH32, TH34, TH35 on WZ50-80]; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller board; 4 Defective thermistor installation (TH3, OCH880).
On the Ecodan R32 Puz Wm
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH6/TH7/TH8/TH34.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan R32 Zubadan Puz Hwm
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH32(suction)/TH34/TH6/TH7/TH8.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH6/TH7/TH8/TH34.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3 (liquid), TH32 (inlet water), TH33/TH34, TH6 (plate HEX liquid), TH7 (ambient), TH8 (heat sink).
When it shows: Open or short detected during compressor operation (open detection of TH3/TH32/TH6 inoperative 10 s-10 min after start and during/10 min after defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Open/short of outdoor unit thermistors (liquid TH3, 2-phase TH6, ambient TH7, heat sink TH8, plate HEX liquid TH34 etc.).
Part involved: Outdoor thermistors TH3/TH6/TH7/TH8/TH34.
When it shows: Open or short detected during compressor operation (TH3/TH6/TH7/TH34: open <= -40C, short >= 90C; TH8: open <= -35C, short >= 102C); open detection of TH3/TH34/TH6 inoperative 10 s-10 min after start and during/10 min after defrost.
Likely causes:
- 1 Disconnection or contact failure of connectors (controller board TH3, TH34, TH7/6; power board CN3); 2 Defective thermistor; 3 Defective outdoor controller circuit board.
Is it safe?
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: Outdoor error cancelled by switching the cylinder unit earth leakage circuit breaker OFF, waiting until the outdoor board LEDs go out (about 1 minute), then ON again (service manual GHH15010).
On the Ecodan Hydrodan
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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 Sw Split
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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 Co2 Quhz
- 1 Connect CN641; 2 Check thermistor resistance/temperature, replace thermistor; 3 Replace outdoor inverter PCB.
- 1 Connect CN643; 2 Check thermistor resistance/temperature, replace thermistor; 3 Replace outdoor inverter PCB.
- 1 Connect CN664; 2 Check thermistor resistance/temperature, replace thermistor; 3 Replace outdoor inverter PCB.
- 1 Replace the outdoor inverter PCB.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Hydrodan
- 1 Check connectors and lead wires; 2 Check resistance or temperature via microprocessor (SW2, PAC-SK52ST); 3 Replace C.B. Emergency operation available for TH3 and TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz Sw Split
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz W Monobloc
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R290 Puz Wz
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Puz Wm
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Zubadan Puz Hwm
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Ultra Quiet Puhz W Vaa
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Shw Split
- 1 Check thermistor connectors on controller board and CN3 on power board, check lead wires; 2 Check thermistor resistance or temperature via SW2 / A-Control service tool (PAC-SK52ST) to identify which thermistor; 3 Replace outdoor controller board. Emergency operation available for TH3, TH34, TH6, TH7 faults.
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.
- Insufficient heating (house not warm enough) on the Ecodan Zubadan Puhz Hw Monobloc: 1 report
Sources
- Ecodan QUHZ-W40VA Service Manual (GHH15010) ↗ · GHH15010 · page 14 · Reviewed 23 September 2026
- 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