JC on a Daikin Altherma HT: Malfunction of low pressure sensor IU/OU
Short answer
On a Daikin heat pump, JC means: Malfunction of low pressure sensor IU/OU. 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 low pressure sensor IU/OU.
Part involved: thermistor / pressure sensor.
When it shows: When the low pressure sensor is short circuit or open circuit.
Likely causes:
- Defect of low pressure sensor; Connection of high pressure sensor with wrong connection; Defect of outdoor unit PC board; Defect of indoor unit PC board (A4P).
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 defect of low pressure sensor; check connection of high pressure sensor with wrong connection; check defect of outdoor unit PC board; 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.
What you might notice
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 190 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026