P4 on a Daikin Altherma LT: Radiation fin thermistor or related abnormality
Short answer
On a Daikin heat pump, P4 means: Radiation fin thermistor or related abnormality. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Radiation fin thermistor or related abnormality.
Part involved: thermistor / pressure sensor.
When it shows: The thermistor input is above 4.96 V or below 0.04 V with the power on. Error J3 is judged if the discharge pipe thermistor temperature is smaller than the condenser thermistor temperature | The thermistor input is above 4.96 V or below 0.04 V with the power on. Error J3 is judged if the discharge pipe thermistor temperature is smaller than the condenser thermistor temperature. 3 3.
Likely causes:
- Connector in poor contact; Thermistor defective; Outdoor unit PCB defective; Hydro-box PCB defective; Condenser thermistor defective in the case of J3 error (outdoor unit heat exchanger thermistor in the cooling mode, or hydro-box heat exchanger thermistor in the heating mode); Condenser thermistor defective in the case of J3 error (outdoor unit heat exchanger; thermistor in the cooling mode, or hydro-box heat exchanger thermistor in the heating mode).
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
What the engineer will check
- Follow the service-manual troubleshooting flow chart for this code: check connector in poor contact; check thermistor defective; check outdoor unit PCB defective; check hydro-box PCB defective; check condenser thermistor defective in the case of J3 error (outdoor unit heat exchanger thermistor in the cooling mode, or hydro-box heat exchanger thermistor in the heating mode); check condenser thermistor defective in the case of J3 error (outdoor unit heat exchanger.
Who to ask for: Heat pump engineer (Daikin-trained).
Certification: Electrical competence to BS 7671 for sensor/PCB wiring; F-gas if a refrigerant-side sensor must be replaced.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 177 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026