L4 on a Daikin heat pump: Malfunction of inverter radiating fin temperature rise
Short answer
On a Daikin heat pump, L4 means: Malfunction of inverter radiating fin temperature rise. 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 inverter radiating fin temperature rise.
Part involved: inverter PCB thermal protection.
When it shows: When the temperature of the inverter radiation fin increases above 83°C | Faulty thermistor. Broken wire.
Likely causes:
- Actuation of fin thermal (Actuates above 83°C); Defect of inverter PC board (A2P); Defect of fin thermistor; Defect of inverter PC board [A4P (1 ø) or A5P (3ø)]; Defect of fin thermistor (R8T).
On the Altherma LT (low temperature split)
The manual says: Radiation fin (power transistor) temperature rise.
Part involved: inverter PCB thermal protection.
When it shows: ■ If the radiation fin temperature with the compressor on is above 105°C, ■ The error is cleared when the temperature drops below 99°C. ■ If a radiation fin temperature rise takes place 4 times successively, the system will be shut down. ■ The error counter will reset itself if this or any other error does not occur during the following 60-minute compressor running time (total time) | If the radiation fin temperature with the compressor on is above 105°C, The error is cleared when the temperature drops below 99°C. If a radiation fin temperature rise takes place 4 times successively, the system will be shut down. 3 3 The error counter will reset itself if this or any other error does not occur during the following 60-minute compressor running time (total time).
Likely causes:
- Fin temperature rise due to defective outdoor unit fan; Fin temperature rise due to short-circuit; Fin thermistor defective; Connector in poor contact.
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? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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.
- Check the heat pump isolator switch(es) and the consumer-unit breaker/RCD for the heat pump are ON. If a breaker or RCD has tripped, do not keep resetting it - leave it and call an engineer.
- Look at the outdoor unit from outside: check the air inlet/outlet and fan grille are not blocked by leaves, debris, snow or heavy ice build-up (do not remove panels or touch the fan).
What the engineer will check
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check actuation of fin thermal (Actuates above 83°C); check defect of inverter PC board (A2P); check defect of fin thermistor; check defect of inverter PC board [A4P (1 ø) or A5P (3ø)]; check defect of fin thermistor (R8T).
Who to ask for: F-gas certified heat pump engineer with electrical competence.
Certification: UK F-gas certification if the refrigerant circuit is opened; electrical competence (BS 7671) for inverter/PCB work.
On the Altherma LT (low temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check fin temperature rise due to defective outdoor unit fan; check fin temperature rise due to short-circuit; check fin thermistor defective; check connector in poor contact.
Who to ask for: F-gas certified heat pump engineer with electrical competence.
Certification: UK F-gas certification if the refrigerant circuit is opened; electrical competence (BS 7671) for inverter/PCB work.
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 194 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 181 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026