P9 on a Mitsubishi Electric heat pump: Thermistor (Tank water temp.) (THW5 / TH5) failure - actual tank temperature sensor error
Short answer
On a Mitsubishi Electric heat pump, P9 means: Thermistor (Tank water temp.) (THW5 / TH5) failure - actual tank temperature 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 · 2 sources
What it means
On the Ecodan
The manual says: Thermistor (Tank water temp.) (THW5 / TH5) failure - actual tank temperature sensor error.
Part involved: DHW tank thermistor THW5/TH5.
When it shows: Tank thermistor open/short (FTC2B/2BR: LED4 blink, LED5 off).
Likely causes:
- Thermistor open/short or disconnected; FTC PCB fault.
On the Ecodan Puhz Sw Split
The manual says: Actual tank temperature thermistor (TH5/THW5) abnormality (open/short) - DHW tank thermistor connected to the FTC/interface.
Part involved: DHW tank thermistor TH5/THW5 (on FTC).
When it shows: Short/open detected: unit enters 3-minute restart-prevention; abnormal if it does not reset normally after 3 minutes; detected in cooling, heating, heating ECO, anti-freeze and hot water operation.
Likely causes:
- 1 Defective thermistor characteristics; 2 Contact failure at interface unit/FTC connector; 3 Wire breakage or contact failure of thermistor wiring; 4 Defective PCB of interface unit/FTC.
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 Puhz Sw Split
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: Unit retries after 3-minute restart prevention; returns to normal operation if reset normally.
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
- Check resistance across the thermistor (0C 15.0 kOhm, 20C 6.3 kOhm, 40C 3.0 kOhm per outdoor service manuals); check connection; replace PCB if reading abnormal.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Puhz Sw Split
- 1-3 Check thermistor resistance (0C 15.0 kOhm, 10C 9.6, 20C 6.3, 30C 4.3, 40C 3.0) while flexing lead; 2 Reseat connector and power-cycle; 4 Compare tank temperature on remote controller, replace FTC PCB if abnormal.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
What you might notice
What owners and engineers report
10 owner and engineer reports mention this.
- No hot water on the Ecodan: 9 reports
- Cylinder or buffer thermostat/sensor problem on the Ecodan: 1 report
Sources
- Ecodan FTC2BR PAC-IF033B-E Installation Manual (BH79D852H01) ↗ · BH79D852H01 · page EN-22 (6.2 Self-check) · Reviewed 23 September 2026
- Ecodan PUHZ-SW(75-100-120)(V-Y)HA-BS Service Manual (OCH755) ↗ · OCH755 · page OCH755 p.30; OCH525C p.28 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026