L4-00 on a Daikin Altherma: OU: Malfunction of inverter radiating fin temperature rise
Short answer
On a Daikin heat pump, L4-00 means: OU: Malfunction of inverter radiating fin temperature rise. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: OU: Malfunction of inverter radiating fin temperature rise.
Part involved: inverter PCB thermal protection.
When it shows: Radiating fin thermistor measures a too high temperature.
Likely causes:
- Faulty outdoor unit fan motor.; ▪ Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%), ▪ Power drop, ▪ Short circuit.; Faulty main PCB. Prerequisite: Stop the unit operation via the user interface. Prerequisite: Turn OFF the respective circuit breaker.; Thermal interface grease NOT applied properly on the heat sink.
Is it safe?
| 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: no.
Manual reset: Manual reset via user interface.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) 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).
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Perform a check of the outdoor unit fan motor. . ; Check if the power supply is compliant with the regulations. . ; Perform a check of the main PCB. . ; Wait until the rectifier voltage is below 10 V DC. DANGER: RISK OF ELECTROCUTION Confirm the rectifier voltage is below 10 V DC before proceeding, see " To prevent electrical hazards " [4 254 ] . ; Check that the thermal interface grease is applied properly on the (PCB or refrigerant piping) contact surface of the heat sink. Adjust if needed.
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
9 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Keeps stopping / cutting out on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 63 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026