P1 on a Mitsubishi Electric heat pump: Flow water (TH1) sensor error
Short answer
On a Mitsubishi Electric heat pump, P1 means: Flow water (TH1) sensor error. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 9 sources
What it means
On the Ecodan
The manuals for this range word it in different ways:
- Flow water (TH1) sensor error.
- Thermistor (Flow water temp.) (THW1) failure.
- Thermistor (Room temp.) (TH1) failure (TH1A/TH1B room temperature thermistor).
Part involved: Flow water thermistor TH1; Flow water thermistor THW1; Room temperature thermistor TH1 (in main remote controller / wireless remote controller / room sensor).
When it shows: Thermistor open (-39C or below) or short (88.5C or above).
Likely causes:
- Thermistor open/short or disconnected; FTC board fault.
- 1 Connector/terminal wire detached or loose; 2 Thermistor fault; 3 FTC board failure; 4 Thermistor in the wireless or main remote controller defective (when room temperature control uses main RC or Room RC 1-8); 5 Incorrect DIP switch setting.
On the Ecodan Co2 Quhz
The manual says: Indoor unit temperature thermistor (room temp. TH1) failure.
Part involved: Room thermistor TH1A/TH1B.
When it shows: Thermistor open or short.
Likely causes:
- Connector detached; thermistor fault; FTC board failure; wireless/main remote controller thermistor defective.
On the Ecodan Hydrodan
The manual says: Thermistor (Room temp.) (TH1) failure (TH1A/TH1B room temperature thermistor).
Part involved: Room temperature thermistor TH1 (in main remote controller / wireless remote controller / room sensor).
When it shows: Thermistor open (-39C or below) or short (88.5C or above).
Likely causes:
- 1 Connector/terminal wire detached or loose; 2 Thermistor fault; 3 FTC board failure; 4 Thermistor in the wireless or main remote controller defective (when room temperature control uses main RC or Room RC 1-8); 5 Incorrect DIP switch setting.
Is it safe?
On the Ecodan
| 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-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 Co2 Quhz
| 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.
Manual reset: Press F4 (Reset) then F3 (Yes) on the main remote controller, or switch system off (installation manual).
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-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.
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
- Check resistance across the thermistor; check connections.
- 1 Check connections; 2 Check thermistor resistance; 3 Replace board; 4 Replace wireless or main remote controller; 5 Check DIP switches / room sensor settings.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Co2 Quhz
- Check connections and resistance; replace board or remote controller; check room sensor settings.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Hydrodan
- 1 Check connections; 2 Check thermistor resistance; 3 Replace board; 4 Replace wireless or main remote controller; 5 Check DIP switches / room sensor settings.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
What owners and engineers report
23 owner and engineer reports mention this.
- Controller / thermostat problem on the Ecodan: 14 reports
- No heating on the Ecodan: 9 reports
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 53 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan FTC5 - EHPT20X-MHCW, EHST20(D)(C)-MHCW Cylinder Service Manual (OCH570C) ↗ · OCH570C · page 64 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MHEDWR1 Service Manual (OCH714E) ↗ · OCH714E · page 56 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 60 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan EHPT20Q VM2EA Service Manual (GCH15010C) ↗ · GCH15010C · page 38 (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
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026