J3 on a Daikin heat pump: Malfunction of discharge pipe thermistor (R2T, R6T) (loose connection, disconnection, short circuit, failure)
Short answer
On a Daikin heat pump, J3 means: Malfunction of discharge pipe thermistor (R2T, R6T) (loose connection, disconnection, short circuit, failure). The manual does not say whether the heat pump stops. 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 Altherma HT (high temperature split)
The manual says: Malfunction of discharge pipe thermistor (R2T, R6T) (loose connection, disconnection, short circuit, failure).
Part involved: thermistor / pressure sensor.
When it shows: When a short circuit or an open circuit in the di scharge pipe temperature thermistor is detected.
Likely causes:
- Defect of thermistor (R2T) for outdoor unit discharge pipe; Defect of outdoor unit PC board (A1P); Defect of thermistor (R6T) for indoor unit discharge pipe; Defect of indoor unit PC board (A4P).
On the Altherma LT (low temperature split)
The manual says: Discharge pipe thermistor failure.
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?
On the Altherma HT (high temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
On the Altherma LT (low temperature split)
| 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
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check defect of thermistor (R2T) for outdoor unit discharge pipe; check defect of outdoor unit PC board (A1P); check defect of thermistor (R6T) for indoor unit discharge pipe; check defect of indoor unit PC board (A4P).
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.
On the Altherma LT (low temperature split)
- 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 Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 181 · Reviewed 23 September 2026
- 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