- Heat pumps
- Air-to-water heat pump
L4-01
Daikin Altherma 4 H F L4-01: inverter radiation fin overheating
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Code L4-01 on a Daikin Altherma 4 H F heat pump means an outdoor unit inverter radiating fin temperature rise fault (overheated radiation fin), stopping the heat pump with a manual reset. Check [11] Malfunction history for P4-01, note [6.3] readings, and call your installer.
Safety first
Stop and check this first
- Do not open the outdoor unit to inspect the inverter radiation fin or PCB yourself: the outdoor unit contains flammable R290 refrigerant and high-voltage capacitors that require disconnecting the power supply for more than 10 minutes and measuring less than 50 V DC before touching electrical parts.
- Never supply power to the heat pump through an external switching device such as a timer or a utility circuit that regularly switches ON and OFF: Daikin prohibits this to prevent hazards from inadvertent resetting of the thermal cut-out.
Safe checks you can perform
- Check [11] Malfunctioning and [11] Malfunction history for P4-01 or other L4/L8 codes
- Read the outside temperature in [6.3] Sensors and active mode in [6.5]
- Inspect the area around the outdoor unit from outside and check [5.23] Emergency selection
Leave these tasks to a qualified professional
- Do not attempt this professional task: Correct the cooling or PCB issue, manually reset L4-01, and verify in [7.3] Operation test run
- Do not attempt this professional task: Measure inverter radiation fin temperature and test fin thermistor FINTh per service manual
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Correct the cooling or PCB issue, manually reset L4-01, and verify in [7.3] Operation test run
What they doOnce heat dissipation on the inverter radiation fin or the defective component is restored and all covers are closed, the installer performs the manual reset of L4-01 and executes [7.3] Maintenance mode > Operation test run through a full compressor cycle.
They must show youthe [11] Malfunctioning and [11] Malfunction history screens free of new L4-01 or P4-01 entries after the [7.3] test run.
In writingthe root cause resolved, the manual reset and [7.3] test run completion, and confirmation that Maintenance mode was exited.
Measure inverter radiation fin temperature and test fin thermistor FINTh per service manual
What they doAfter disconnecting mains power for more than 10 minutes and verifying capacitor voltage is below 50 V DC, the authorised Daikin technician inspects the inverter radiation fin heat-transfer path, checks the fin thermistor (FINTh) resistance and connector against service manual values, and checks live fin temperature readout during operation.
They must show youthe measured fin thermistor resistance and connector condition alongside the live fin temperature reading compared with the service manual.
In writingwhether L4-01 was caused by actual radiation fin overheating or a fin thermistor fault, plus any part reference replaced.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Check [11] Malfunctioning and [11] Malfunction history for P4-01 or other L4/L8 codes
- Who:
- You, viewing L4-01 on the indoor touchscreen with advanced end user permission enabled.
- How:
- Open [11] Malfunctioning and tap L4-01 to view its long description, then open [11] Malfunction history to check whether P4-01 (Fin thermistor abnormality), L4-00, L4-06, L4-07, or L8 thermal protection codes were logged near the same timestamp.
- Result that matters:
- Determining whether the controller also flagged a fin thermistor sensor abnormality (P4-01) or multiple inverter thermal trips.
- If P4-01 appears in Malfunction history alongside L4-01, that points to a fin thermistor or sensor wiring abnormality that the technician should test before suspecting real inverter overheating.
- If L4-01 appears without P4-01 (or alongside L4-00 or L8 codes), that points to elevated temperature at the inverter radiation fin for the technician to inspect.
- Evidence to keep:
- Photograph the L4-01 long description screen and the timestamped list in [11] Malfunction history.
Do not attempt to reset L4-01 repeatedly on your own; Daikin states it is important to understand the problem and take measures before resetting an error code.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Read the outside temperature in [6.3] Sensors and active mode in [6.5]
- Who:
- You, reading out sensor and mode values from menu [6] Information on the indoor display.
- How:
- Open [6.3] Information > Sensors and [6.5] Information > Operation modes to record the outside ambient temperature and active mode, comparing them with Daikin's outdoor ambient range (-28~25°C heating, 10~43°C cooling, up to 40°C domestic hot water).
- Result that matters:
- Whether L4-01 occurred while the outdoor unit was running beyond its published ambient temperature limits.
- If the outside temperature was above the published limit for the active mode, that indicates high ambient heat stress contributed to the inverter radiation fin overheating.
- If the outside temperature was well within design limits, that rules out ambient weather extremes and isolates the issue to local cooling of the radiation fin, fin thermistor FINTh, or the inverter stage.
- Evidence to keep:
- Save a photo of the [6.3] Sensors readout and write down the active mode from [6.5].
Never open the outdoor unit casing or touch the inverter radiation fin yourself.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Inspect the area around the outdoor unit from outside and check [5.23] Emergency selection
- Who:
- You, visually checking the outdoor unit exterior and the indoor user interface.
- How:
- Confirm from outside that the space around the outdoor unit is clean, unenclosed, and free of objects resting on the casing, then open [5.23] Settings > Emergency selection to see whether the electrical heater takes over space heating and DHW while the heat pump is stopped.
- Result that matters:
- Eliminating obvious external airflow blockages and ensuring backup heating or hot water continuity until the installer arrives.
- If something was blocking the outdoor unit airflow or enclosing the unit, that points to heat recirculation around the outdoor unit that you should remove from outside and report.
- If the outdoor unit is unobstructed and [5.23] Emergency selection enables electrical heater takeover, that confirms backup heating will cover the house (with higher electricity consumption) until professional repair.
- Evidence to keep:
- Keep a photo of the outdoor unit surroundings and a photo of the [5.23] Emergency selection screen.
Do not turn off the unit circuit breakers while waiting for service; continuous power is required to keep R290 leak detection and freeze protection active.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Daikin
- Product type
- Air-to-water heat pump
Known code variants
- Altherma 4 H F L4-01
- code erreur Daikin Altherma L4-01
- codice L4-01 pompa di calore Daikin
- código L4-01 bomba de calor Daikin
- Daikin Altherma error L4-01
- Daikin Altherma Fehler L4-01
- Daikin heat pump code L4-01
- Daikin L4-01
- Daikin Wärmepumpe Fehlercode L4-01
- défaut L4-01 pompe à chaleur Daikin
- error Daikin Altherma L4-01
- errore Daikin Altherma L4-01
What it means
In Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 210), error code L4-01 is titled OU: Malfunction of inverter radiating fin temperature rise with the trigger Overheating of inverter radiation fin, stopping heat pump operation under a manual reset. Table 14.4.4 assigns the identical title, trigger, effect, and manual reset to both L4-00 and L4-01, while L4-06 and L4-07 cover fan 1 and fan 2 radiation fins and P4-01 covers Fin thermistor abnormality (FINTh in the W1 wiring diagram). Because impaired heat dissipation or a faulty fin thermistor reading can trigger L4-01, the code alone does not prove the inverter PCB is damaged.
Warnings and stop conditions
- Highsource verified
Do not open the outdoor unit to inspect the inverter radiation fin or PCB yourself: the outdoor unit contains flammable R290 refrigerant and high-voltage capacitors that require disconnecting the power supply for more than 10 minutes and measuring less than 50 V DC before touching electrical parts.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Mediumsource verified
Never supply power to the heat pump through an external switching device such as a timer or a utility circuit that regularly switches ON and OFF: Daikin prohibits this to prevent hazards from inadvertent resetting of the thermal cut-out.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Actual overheating of the outdoor unit inverter radiation finPossible
Insufficient heat rejection from the inverter radiation fin due to obstructed outdoor airflow, high ambient temperatures outside the operating window, or heavy inverter load pushes the fin temperature above its limit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Inaccurate temperature readout from the fin thermistor (FINTh / P4-01)Possible
A drift, poor connector contact, or fault in the inverter radiation fin thermistor (which has its own error code P4-01, Fin thermistor abnormality) can cause the inverter PCB to register a false overheating condition.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does L4-01 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Daikin Europe N.V. · 2026 · official_doc
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