C4 on a Daikin heat pump: Malfunction of thermistor (R3T) for R-410A liquid/indoor unit (loose connection, disconnection, short circuit, failure)
Short answer
On a Daikin heat pump, C4 means: Malfunction of thermistor (R3T) for R-410A liquid/indoor unit (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 thermistor (R3T) for R-410A liquid/indoor unit (loose connection, disconnection, short circuit, failure).
Part involved: thermistor / pressure sensor.
When it shows: When the liquid thermistor becomes disconnect ed or shorted while the unit is running.
Likely causes:
- Defect of thermistor (R3T) for liquid pipe; Defect of indoor unit PC board (A1P).
On the Altherma LT (low temperature split)
The manual says: Heat exchanger thermistor abnormality.
Part involved: thermistor / pressure sensor.
When it shows: When the thermistor input is more than 4.96 V or less than 0.04 V during compressor operation*. * (reference) When above about 212°C (less than 120 ohms) or below about –50°C (more than 1,860 kohms). Note: The values vary slightly in some models.
Likely causes:
- Faulty connector connection; Faulty thermistor; Faulty PCB.
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 (R3T) for liquid pipe; check defect of indoor unit PC board (A1P).
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 faulty connector connection; check faulty thermistor; check faulty PCB.
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 154 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 150 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026