U3 on a Mitsubishi Electric heat pump: Short circuit or disconnection of discharge temperature thermistor (TH1)
Short answer
On a Mitsubishi Electric heat pump, U3 means: Short circuit or disconnection of discharge temperature thermistor (TH1). 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 manual says: Short circuit or disconnection of discharge temperature thermistor (TH1).
Part involved: Discharge thermistor TH1 (CN641).
When it shows: Disconnection: -5C or less for 2 min from 10 min after compressor start; short: continued 2 min at all times.
Likely causes:
- 1 Defective connector CN641 connection; 2 TH1 disconnection or short circuit; 3 Defective outdoor inverter PCB.
On the Ecodan Hydrodan
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (3C or less) or short (217C or more) during compressor operation.
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Puhz Sw Split
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (-20C or less) or short (217C or more) detected during compressor operation (open detection inoperative 10 min from start and 10 min after/during defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Puhz W Monobloc
The manual says: Open/short circuit of discharge temperature thermistor (TH4) / compressor surface thermistor (TH33 or TH34 depending on model).
Part involved: Thermistors TH4, TH33.
When it shows: Open (3C or less) or short (217C or more) during compressor operation (open detection inoperative 10 min from start and during/10 min after defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan R290 Puz Wz
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (-20C or less) or short (217C or more) detected during compressor operation (open detection inoperative 10 min from start and 10 min after/during defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan R32 Puz Wm
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (-20C or less) or short (217C or more) detected during compressor operation (open detection inoperative 10 min from start and 10 min after/during defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan R32 Zubadan Puz Hwm
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (3C or less) or short (217C or more) during compressor operation.
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Ultra Quiet Puhz W Vaa
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (-20C or less) or short (217C or more) detected during compressor operation (open detection inoperative 10 min from start and 10 min after/during defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Zubadan Puhz Hw Monobloc
The manual says: Open/short circuit of discharge temperature thermistor (TH4) / compressor surface thermistor (TH33 or TH34 depending on model).
Part involved: Thermistors TH4, TH33.
When it shows: Open (3C or less) or short (217C or more) during compressor operation (open detection inoperative 10 min from start and during/10 min after defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 2 Defective thermistor; 3 Defective outdoor controller circuit board.
On the Ecodan Zubadan Puhz Shw Split
The manual says: Open/short circuit of outdoor unit temperature thermistor (discharge TH4 / compressor surface TH33).
Part involved: Thermistors TH4, TH33.
When it shows: Open (-20C or less) or short (217C or more) detected during compressor operation (open detection inoperative 10 min from start and 10 min after/during defrost).
Likely causes:
- 1 Disconnection or contact failure of connectors (TH4, TH33) on outdoor controller circuit board; 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? | 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 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.
What the engineer will check
On the Ecodan Co2 Quhz
- 1 Connect CN641; 2 Check TH1 resistance/temperature, replace TH1; 3 Replace outdoor inverter PCB.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Hydrodan
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz Sw Split
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz W Monobloc
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R290 Puz Wz
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Puz Wm
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan R32 Zubadan Puz Hwm
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 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 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Hw Monobloc
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Zubadan Puhz Shw Split
- 1 Check connector (TH4, TH33) and lead wires; 2 Check thermistor resistance or temperature via microprocessor (SW2 / A-Control service tool); 3 Replace outdoor controller board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
Sources
- Ecodan QUHZ-W40VA Service Manual (GHH15010) ↗ · GHH15010 · page 13 · 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.24; OCH525C p.22 · Reviewed 23 September 2026
- Ecodan PUHZ-W112VHA-BS Service Manual (OCH562D) ↗ · OCH562D · page OCH562D p.19; OCH605A p.16; OCH465D p.21; OCH439N p.55 · Reviewed 23 September 2026
- PUZ-WZ(85-100-120)(V-Y)AA(-BS) Service_Manual (OCH880) ↗ · OCH880 · page OCH880 p.38-39; OCH816 p.34 · Reviewed 23 September 2026
- Ecodan PUZ-WM(50-60-85-112)V(H-A)A Service Manual (OCH727A) ↗ · OCH727A · page 25 · Reviewed 23 September 2026
- Ecodan PUZ-HWM140(V-Y)HA Service Manual (OCH748A) ↗ · OCH748A · page 22 · Reviewed 23 September 2026
- PUHZ-W85-112VAA(-BS) Service Manual (OCH681) ↗ · OCH681 · page 16 · Reviewed 23 September 2026
- Ecodan PUHZ-HW140VHA2-BS / PUHZ-HW140YHA2-BS Service Manual (OCH439N) ↗ · OCH439N · page 55 · Reviewed 23 September 2026
- PUHZ-SHW112-140V/YHA Service Manual (OCH526) ↗ · OCH526 · page 26 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026