- Heat pumps
- Air-to-water heat pump
U2-37
Daikin Altherma U2-37 outdoor fan2 supply voltage fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma 4 H F shows U2-37 when it detects a power supply voltage error on outdoor fan2 (OU fan2), halting the heat pump until a manual reset is performed. Check the error text in menu [11] Malfunctioning and inspect the outdoor fan2 grille from outside without opening any panel.
Safety first
Stop and check this first
- Do not open the outdoor unit casing or switch box yourself: lethal residual voltage remains on internal capacitors until the power supply has been disconnected for more than 10 minutes and verified below 50 V DC, and opening the unit can impair the R290 leak sensor.
- Do not repeatedly reset U2-37 or bypass any safety device to force the heat pump to run without fixing the OU fan2 supply voltage fault.
Safe checks you can perform
- Review the U2-37 error screen and compare recent entries in menu [11] Malfunction history
- Visually inspect the outdoor fan2 grille and unit surroundings without removing panels
- Confirm emergency heating preferences in [5.23] and retrieve dealer details in [6.2]
Leave these tasks to a qualified professional
- Do not attempt this professional task: Isolate mains power, verify capacitor discharge below 50 V DC, and inspect the outdoor switch box and fan2 harness
- Do not attempt this professional task: Test the OU fan2 power supply rail, fan2 motor, and outdoor PCB per the service manual before manual reset
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.
Isolate mains power, verify capacitor discharge below 50 V DC, and inspect the outdoor switch box and fan2 harness
What they doA qualified technician must turn off the mains circuit breakers, wait more than 10 minutes, verify with a multimeter that capacitor voltage is below 50 V DC, and perform a thorough visual inspection of the switch box and OU fan2 wiring for loose connections or damaged insulation.
They must show youThe measured DC bus discharge reading below 50 V DC, the incoming AC supply voltage vs the identification label, and the physical condition of the fan2 plug and switch box terminals..
In writingThe recorded AC supply voltage, DC discharge verification, and wiring inspection findings on the maintenance report..
Test the OU fan2 power supply rail, fan2 motor, and outdoor PCB per the service manual before manual reset
What they doUsing the model service manual procedure for U2-37, the technician must measure the supply voltage output to outdoor fan2 and test the fan2 motor and outdoor PCB so that the root cause is corrected before executing the manual reset.
They must show youThe multimeter readings on the OU fan2 supply connector and motor windings compared with service manual reference values, followed by smooth dual-fan operation during a test run..
In writingThe measured fan2 supply voltage, motor test values, any replaced component serial numbers, and confirmation of fault-free operation after manual reset..
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.
Review the U2-37 error screen and compare recent entries in menu [11] Malfunction history
- Who:
- You or an advanced end user operating the indoor user interface.
- How:
- Open [11] Malfunctioning and tap the U2-37 alert to display the built-in help message, then switch to [11] Malfunction history to check whether U2-36 (OU fan1) or U2-43/U2-44 also registered.
- Result that matters:
- Whether U2-37 is the sole fault recorded or coincides with U2-36 or other supply voltage errors.
- If U2-37 appears alongside U2-36 or another U2 supply fault, that points to a common outdoor supply voltage disturbance or main board rail issue rather than an isolated fan2 defect.
- If U2-37 is the only code in the malfunction history, that narrows the fault to the OU fan2 branch, its connector harness, or the second fan assembly.
- Evidence to keep:
- A screenshot or photo of the [11] Malfunctioning help text and the timestamped list 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
Visually inspect the outdoor fan2 grille and unit surroundings without removing panels
- Who:
- You, observing the outdoor unit from outside with all covers closed.
- How:
- Check the fan2 guard and the perimeter around the outdoor unit for leaves, snow drift, frost bridging, or objects blocking airflow.
- Result that matters:
- Whether the area around the unit is clean and the fan2 blades sit free of visible foreign material behind the grille.
- If snow, ice, or garden debris is pressing through the fan2 guard, that indicates an external mechanical obstruction that must be safely cleared after professional isolation.
- If the fan2 opening and outdoor perimeter are completely clear, that rules out external fouling and points to an electrical supply or internal fan2 circuit fault.
- Evidence to keep:
- A clear photo of the outdoor unit fan grilles and surrounding clearance.
Evidence: Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Confirm emergency heating preferences in [5.23] and retrieve dealer details in [6.2]
- Who:
- You, at the indoor MMI controller while the heat pump is stopped by U2-37.
- How:
- Because U2-37 stops the heat pump until a manual reset, check the emergency operation pop-up or go to [5.23] Settings > Emergency selection to manage backup heater support, then open [6.2] Information > Dealer information.
- Result that matters:
- Confirmed backup heater behavior for space heating and domestic hot water, plus the service contact phone number.
- If the indoor unit responds normally and runs emergency backup heating on request, that confirms the indoor power circuit and user interface are healthy while the outdoor unit awaits service.
- If the indoor screen shows additional supply warnings or fails to energize backup heating, that suggests a broader building power distribution problem.
- Evidence to keep:
- A note of the active [5.23] Emergency selection mode and the timestamp when U2-37 halted the heat pump.
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 U2-37
- code erreur Daikin Altherma U2-37
- codice U2-37 pompa di calore Daikin
- código U2-37 bomba de calor Daikin
- Daikin Altherma error U2-37
- Daikin Altherma Fehler U2-37
- Daikin heat pump code U2-37
- Daikin U2-37
- Daikin Wärmepumpe Fehlercode U2-37
- défaut U2-37 pompe à chaleur Daikin
- error Daikin Altherma U2-37
- errore Daikin Altherma U2-37
What it means
In the Daikin Altherma 4 H F installer reference guide, error code U2-37 is listed under Supply voltage error as an OU fan2 power supply voltage error, causing the heat pump to stop operating until a manual reset. On outdoor units equipped with a second fan circuit, the controller monitors the supply voltage feeding outdoor fan2 during startup and operation. Because U2-37 can arise from an unstable outdoor mains supply, a loose connector or chafed wire on the fan2 harness, external debris stalling the fan2 impeller, or an electrical fault on the outdoor PCB or fan2 motor, the error code by itself never proves that the fan2 motor or board must be replaced.
Warnings and stop conditions
- Highsource verified
Do not open the outdoor unit casing or switch box yourself: lethal residual voltage remains on internal capacitors until the power supply has been disconnected for more than 10 minutes and verified below 50 V DC, and opening the unit can impair the R290 leak sensor.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Mediumsource verified
Do not repeatedly reset U2-37 or bypass any safety device to force the heat pump to run without fixing the OU fan2 supply voltage fault.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Outdoor supply voltage instability affecting the OU fan2 circuitCommon
Fluctuating mains supply voltage or a poor electrical connection feeding the outdoor unit disturbed the voltage rail supplying the OU fan2 circuit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose plug or damaged lead on the outdoor fan2 wiring harnessPossible
A loose plug, pinched lead, or corroded contact along the wiring harness between the outdoor printed circuit board and outdoor fan2 interrupted its supply voltage.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Snow, ice, or foliage blocking the outdoor fan2 impellerPossible
Ice, foliage, or windblown debris lodged at the fan2 discharge grille mechanically loaded the second fan during operation and triggered a fan2 supply voltage error.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Internal electrical defect in the outdoor fan2 motor or PCB driverLess common
A hardware fault inside the outdoor fan2 motor electronics or on the outdoor PCB fan2 power stage prevented proper voltage delivery.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-37 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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