- Heat pumps
- Air-to-water heat pump
L4-00
Daikin Altherma L4-00: inverter radiation fin overheating
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma L4-00 means the outdoor unit stopped because the inverter radiation fin overheated and locked out on a manual reset. Check that the outdoor coil and fan grille are clear of debris, record menu [11] Malfunctioning, and avoid resetting until a technician inspects the cooling path.
Safety first
Stop and check this first
- Never shunt thermal safety devices or repeatedly force a manual reset on L4-00 without identifying the cause of overheating, as continued operation with an overheated inverter fin can permanently damage the inverter PCB.
- Never open the outdoor switch box cover yourself. Only a qualified technician may open the switch box after turning off all power supplies, waiting more than 10 minutes, and measuring less than 50 V DC on the high-voltage capacitors.
Safe checks you can perform
- Read the long L4-00 description and malfunction history in menu [11]
- Inspect the outdoor unit exterior for blocked grilles, foliage or enclosure traps
- Compare outdoor ambient temperature and sensor readings in menu [6.3]
Leave these tasks to a qualified professional
- Do not attempt this professional task: Test the inverter fin thermistor against P4-01 resistance curves and check inverter PCB current
- Do not attempt this professional task: Perform a 10-minute de-energized capacitor discharge and inspect the inverter radiating fin
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.
Test the inverter fin thermistor against P4-01 resistance curves and check inverter PCB current
What they doThe technician must measure the inverter fin thermistor resistance at ambient temperature, check its harness plug on the inverter PCB, and verify under test operation that neither L4-00 nor DC overcurrent codes (L5-00, L8-00) return.
They must show youThe fin thermistor ohmic reading compared against the temperature chart in the service documentation and the live fin temperature reading during a test run.
In writingMeasured thermistor resistance (ohms) at the recorded ambient temperature (°C), inverter PCB inspection result, and post-reset test run log.
Perform a 10-minute de-energized capacitor discharge and inspect the inverter radiating fin
What they doA Daikin certified technician must turn off all power supplies, wait more than 10 minutes, verify that residual DC link voltage is below 50 V DC before touching electrical parts in the outdoor switch box, and inspect the inverter radiating fin, thermal paste and internal cooling fan passage.
They must show youThe multimeter DC voltage reading below 50 V DC after the 10-minute wait and the physical condition of the inverter radiating fin inside the switch box.
In writingMeasured residual DC bus voltage (V DC), condition of the radiating fin and fan ducting, and any corrective cleaning performed.
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.
Read the long L4-00 description and malfunction history in menu [11]
- Who:
- You (the system owner or user at the indoor user interface).
- How:
- On the indoor controller, open [11] Malfunctioning to read the long error description, and switch user permission to advanced end user to view the malfunction history.
- Result that matters:
- Whether L4-00 is a single isolated event or appears alongside fan or thermistor codes such as L1-55, L3-00, L4-06, L4-07 or P4-01.
- If the history shows companion fan or sensor codes before L4-00, that points to an internal fan driver or fin thermistor fault rather than simple weather load.
- If L4-00 occurred alone during peak afternoon heat or domestic hot water recovery, that suggests thermal saturation or restricted airflow at the outdoor unit.
- Evidence to keep:
- Take a photo of the [11] Malfunctioning screen, the malfunction history log with dates and times, and the dealer contact phone number in [6.2] Dealer information.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Inspect the outdoor unit exterior for blocked grilles, foliage or enclosure traps
- Who:
- You, performing a visual walk-around from outside without opening any panels.
- How:
- Look at the rear coil, side intake and front fan discharge of the outdoor unit to verify that no leaves, snow, tarps or stored items block air circulation.
- Result that matters:
- Clean, unobstructed intake and exhaust paths around the entire outdoor casing.
- If heavy debris or a cover is blocking the outdoor heat exchanger, clearing the exterior obstruction confirms an airflow bottleneck that must be removed before any manual reset.
- If the surrounding area and coil face are completely clear, that rules out external blockage and points to an internal fin cooling or inverter PCB problem.
- Evidence to keep:
- Save a clear photo of the outdoor unit clearances and coil surface condition to show the attending technician.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Compare outdoor ambient temperature and sensor readings in menu [6.3]
- Who:
- You, reading the indoor user interface status screens.
- How:
- Open [6.3] Sensors on the user interface to check current outdoor ambient, water and tank temperatures, and check [5.23] Settings > Emergency selection if heating or hot water is urgently needed.
- Result that matters:
- Whether the outdoor unit was operating within its rated ambient envelope (heating down to -28~25°C, cooling 10~43°C, domestic hot water up to 40°C).
- If outdoor temperature at the unit exceeds 40°C during domestic hot water or 43°C during cooling, that indicates ambient operating limit stress as the trigger.
- If ambient temperature is mild and L4-00 trips shortly after compressor start, that isolates the fault to internal heatsink dissipation or fin temperature sensing.
- Evidence to keep:
- Record the [6.3] sensor readings and the exact time of the lockout before requesting service.
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 L4-00
- code erreur Daikin Altherma L4-00
- codice L4-00 pompa di calore Daikin
- código L4-00 bomba de calor Daikin
- Daikin Altherma error L4-00
- Daikin Altherma Fehler L4-00
- Daikin heat pump code L4-00
- Daikin L4-00
- Daikin Wärmepumpe Fehlercode L4-00
- défaut L4-00 pompe à chaleur Daikin
- error Daikin Altherma L4-00
- errore Daikin Altherma L4-00
What it means
In chapter 14.4.4 of the Daikin Altherma 4 H F installer reference guide (4PEN773391-1C), code L4-00 (and related code L4-01) is titled OU: Malfunction of inverter radiating fin temperature rise, triggered by overheating of the inverter radiation fin. When triggered, the heat pump stops operating and holds a manual reset requirement. Seeing L4-00 on the user interface does not by itself prove the inverter PCB has failed: obstructed outdoor airflow, high ambient load up to 43°C in cooling or 40°C in domestic hot water, a stalled heatsink cooling path, or a drifting fin thermistor (related to P4-01) can all raise the measured fin temperature.
Warnings and stop conditions
- Highsource verified
Never shunt thermal safety devices or repeatedly force a manual reset on L4-00 without identifying the cause of overheating, as continued operation with an overheated inverter fin can permanently damage the inverter PCB.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Highsource verified
Never open the outdoor switch box cover yourself. Only a qualified technician may open the switch box after turning off all power supplies, waiting more than 10 minutes, and measuring less than 50 V DC on the high-voltage capacitors.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Symptoms and causes
Probable causes · in order
- Blocked outdoor airflow or dirty heat exchanger and radiating fin pathCommon
Leaves, snow, packaging or tight clearances around the outdoor unit restrict heat rejection across the inverter radiating fin, driving fin temperature past the L4-00 trip point.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- High outdoor ambient temperature combined with heavy compressor demandPossible
Sustained operation near the upper outdoor ambient boundary (up to 43°C in cooling mode or up to 40°C in domestic hot water production) reduces the thermal margin of the inverter heatsink.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Faulty inverter heatsink cooling fan or degraded thermal contact on the finPossible
An internal cooling fan driver issue (associated with L1-55 or fan fin codes L4-06 and L4-07) or poor thermal transfer between the power module and its radiating fin causes rapid local overheating.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Inverter radiating fin thermistor drift or inverter PCB hardware faultLess common
A shifted resistance curve on the fin thermistor (tracked separately as P4-01 when open or shorted) or an internal short on the inverter PCB reports excessive fin temperature.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does L4-00 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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