J9 on a Daikin Altherma HT: Malfunction of subcooling heat exchanger gas pipe thermistor (R4T)
Short answer
On a Daikin heat pump, J9 means: Malfunction of subcooling heat exchanger gas pipe thermistor (R4T). 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: Malfunction of subcooling heat exchanger gas pipe thermistor (R4T).
Part involved: thermistor / pressure sensor.
When it shows: When the subcooling heat exchanger gas pipe thermistor is short circuited or open.
Likely causes:
- Faulty subcooling heat exchanger gas pipe thermistor (R4T); Faulty outdoor unit PC board.
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.
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 faulty subcooling heat exchanger gas pipe thermistor (R4T); check faulty outdoor unit PC board.
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
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 187 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026