- Heat pumps
- Air-to-water heat pump
U2-36
Daikin Altherma U2-36 outdoor fan1 supply voltage fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma 4 H F displays U2-36 when the outdoor unit detects a power supply voltage error on outdoor fan1 (OU fan1), stopping heat pump operation until a manual reset. Check menu [11] Malfunctioning and inspect the outdoor fan1 grille from outside without opening the casing.
Safety first
Stop and check this first
- Never open the outdoor unit casing or reach into the fan section: always disconnect mains power and wait more than 10 minutes until internal capacitor voltage measures below 50 V DC, and keep the casing closed so the R290 leak sensor functions properly.
- Never reset U2-36 repeatedly without identifying why the OU fan1 power supply voltage error occurred, and never shunt or bypass any safety device on the heat pump.
Safe checks you can perform
- Open the error detail screen and malfunction history in menu [11] on the indoor interface
- Look through the exterior grille of the outdoor unit for obstructions around fan1
- Check whether backup emergency heating is configured in menu [5.23] while the heat pump is stopped
Leave these tasks to a qualified professional
- Do not attempt this professional task: Diagnose the OU fan1 supply voltage rail and motor M1F per the service manual before manual reset
- Do not attempt this professional task: Inspect outdoor switch box connections and verify incoming supply voltage against the nameplate
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.
Diagnose the OU fan1 supply voltage rail and motor M1F per the service manual before manual reset
What they doFollowing the model service manual for U2-36, the technician must measure the supply voltage delivered to outdoor fan1 and test the fan1 motor and outdoor main PCB A1P so only the failed assembly is replaced before performing a manual reset.
They must show youThe measured fan1 supply voltage and winding or connector readings compared against the service manual specification, plus a normal fan1 spin-up after the manual reset..
In writingThe exact voltage and resistance measurements on the OU fan1 circuit, the part number replaced (if any), and confirmation that U2-36 did not return during test operation..
Inspect outdoor switch box connections and verify incoming supply voltage against the nameplate
What they doA qualified technician must disconnect the unit from the mains, wait more than 10 minutes until capacitor voltage is confirmed below 50 V DC, and inspect the outdoor switch box wiring, terminal blocks, and fan1 harness for loose connections or damaged insulation.
They must show youThe multimeter reading confirming residual DC voltage below 50 V DC before work begins, followed by the live AC supply voltage reading compared with the unit identification label..
In writingThe measured incoming AC voltage, the condition of the outdoor switch box terminals and fan1 connector, and confirmation that no safety device was shunted..
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.
Open the error detail screen and malfunction history in menu [11] on the indoor interface
- Who:
- You or an advanced end user at the indoor user interface.
- How:
- Navigate to [11] Malfunctioning, tap the U2-36 message on the error screen to read the guidance, and check [11] Malfunction history to see whether U2-37 or another U2 supply fault occurred at the same time.
- Result that matters:
- Whether U2-36 appears alone for OU fan1 or together with U2-37 (OU fan2) or general supply voltage errors.
- If U2-36 is accompanied by U2-37 or U2-43/U2-44 in the malfunction history, that points to a shared outdoor power supply or main board rail disturbance affecting more than just fan1.
- If only U2-36 is logged while all other supply codes are absent, that isolates the fault to the OU fan1 power supply path, harness, or motor circuit.
- Evidence to keep:
- A clear photograph of the U2-36 error screen and the recent entries in [11] Malfunction history.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Look through the exterior grille of the outdoor unit for obstructions around fan1
- Who:
- You, looking from outside the unit without opening any panel or inserting tools through the fan guard.
- How:
- Inspect the outdoor unit air outlet grille and surrounding perimeter for leaves, snow, ice, or packaging material pressing against the fan1 blades.
- Result that matters:
- Whether the area around the outdoor unit is clean and the fan1 blades appear unobstructed behind the guard.
- If visible foliage, snow buildup, or debris is wedged against the exterior grille, that suggests a physical airflow or blade obstruction contributing to the fan1 fault.
- If the exterior grille is completely clear and nothing touches the fan1 blades, that rules out an external blockage and points to an internal fan1 supply voltage or wiring issue.
- Evidence to keep:
- A photo of the outdoor unit front grille and surrounding clearance for the service technician.
Evidence: Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check whether backup emergency heating is configured in menu [5.23] while the heat pump is stopped
- Who:
- You, using the indoor MMI menu while waiting for a Daikin technician.
- How:
- Because U2-36 stops heat pump operation until a manual reset, open [5.23] Settings > Emergency selection or respond to the emergency pop-up screen to verify whether the backup heater is permitted to maintain indoor comfort or hot water.
- Result that matters:
- Whether the emergency operation mode is set to manual or automatic backup heater activation while the outdoor unit is halted.
- If the backup heater engages via the emergency selection pop-up without tripping any breaker, that confirms the indoor hydrobox power supply remains intact while the outdoor heat pump is stopped by U2-36.
- If the indoor interface also loses power or cannot run emergency operation, that indicates a wider building electrical supply fault affecting the system.
- Evidence to keep:
- A note of the setting selected in [5.23] Emergency selection and the dealer contact details from [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
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 U2-36
- code erreur Daikin Altherma U2-36
- codice U2-36 pompa di calore Daikin
- código U2-36 bomba de calor Daikin
- Daikin Altherma error U2-36
- Daikin Altherma Fehler U2-36
- Daikin heat pump code U2-36
- Daikin U2-36
- Daikin Wärmepumpe Fehlercode U2-36
- défaut U2-36 pompe à chaleur Daikin
- error Daikin Altherma U2-36
- errore Daikin Altherma U2-36
What it means
According to the Daikin Altherma 4 H F installer reference guide, error code U2-36 belongs to the Supply voltage error category and identifies an OU fan1 power supply voltage error, which stops the heat pump and requires a manual reset. On dual-fan outdoor units, fan1 is the primary outdoor fan motor circuit driven and monitored through the outdoor printed circuit board assembly. Because a fan1 supply voltage fault can be triggered by an abnormal mains input feeding the outdoor unit, a loose harness connector inside the electrical box, mechanical binding on the fan1 blade, or a fault on the outdoor PCB or fan motor M1F, the code alone does not prove which part has failed.
Warnings and stop conditions
- Highsource verified
Never open the outdoor unit casing or reach into the fan section: always disconnect mains power and wait more than 10 minutes until internal capacitor voltage measures below 50 V DC, and keep the casing closed so the R290 leak sensor functions properly.
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 reset U2-36 repeatedly without identifying why the OU fan1 power supply voltage error occurred, and never shunt or bypass any safety device on the heat pump.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Outdoor mains supply disturbance affecting the OU fan1 voltage railCommon
Abnormal mains voltage, phase disturbance, or a loose supply connection to the outdoor unit caused the DC bus or supply rail feeding the OU fan1 circuit to move outside its operating window.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose connector or damaged wiring harness to outdoor fan1Possible
A loose plug, damaged wire harness, or oxidized terminal between the outdoor main printed circuit board A1P and the fan1 motor assembly interrupted the fan1 supply voltage.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- External debris or ice obstructing outdoor fan1 rotationPossible
Leaves, ice accumulation, or foreign debris around the outdoor unit blocked fan1 rotation and loaded the fan1 power supply rail until U2-36 latched.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Faulty outdoor fan1 motor M1F or outdoor main PCB A1P driver stageLess common
An internal electrical failure in the fan1 motor driver stage or on the outdoor printed circuit board prevented normal supply voltage regulation for OU fan1.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-36 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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