- Heat pumps
- Air-to-water heat pump
U2-31
Daikin Altherma U2-31 supply voltage error: instant overcurrent
Risk level
High
Professional inspection is recommended.
Urgency
Immediate
Quick answer
Daikin Altherma 4 H F logs U2-31 when its outdoor power circuit detects an instantaneous overcurrent event under the supply voltage error group, while the unit continues operating and resets automatically. Open menu [11] Malfunctioning to note the timestamp and check whether the spike recurs.
Safety first
Stop and check this first
- Do not open the outdoor or indoor unit casing or switch box yourself: high-voltage capacitors require more than 10 minutes after mains disconnection to drop below 50 V DC, and opening the casing can disturb the R290 refrigerant leak detection sensor.
- After commissioning, do not turn off the circuit breakers supplying the heat pump units during normal use, because the R290 leak detection sensor and freeze protection functions require continuous electrical power.
Safe checks you can perform
- Read the U2-31 help screen and malfunction history on the indoor user interface
- Verify at the consumer board that the heat pump circuit is not shared with other appliances
- Inspect the exterior cable route and airflow perimeter around the outdoor unit
Leave these tasks to a qualified professional
- Do not attempt this professional task: Verify dedicated circuit compliance, nameplate voltage match, and switch box terminal tightness
- Do not attempt this professional task: Analyze recurring instantaneous overcurrent triggers across the outdoor inverter and compressor circuit
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.
Verify dedicated circuit compliance, nameplate voltage match, and switch box terminal tightness
What they doHave a qualified electrician or Daikin technician isolate the mains, wait for the high-voltage capacitors to discharge below 50 V DC after more than 10 minutes, and inspect the switch box terminals and feeder wiring for loose connections, spliced extensions, or N-phase faults.
They must show youThe measured supply voltage under standby and compressor load compared against the identification label rating, plus the visual inspection of the switch box terminals..
In writingThe recorded phase-to-neutral and phase-to-phase voltages, confirmation of a dedicated circuit with intact neutral, and terminal torque verification on the service sheet..
Analyze recurring instantaneous overcurrent triggers across the outdoor inverter and compressor circuit
What they doIf the external supply and switch box connections pass inspection but U2-31 keeps logging in the malfunction history, the Daikin certified technician must follow the model service manual procedure for U2-31 to measure running current waveforms and test the outdoor main PCB A1P and compressor load.
They must show youThe malfunction history log from menu [11] alongside the clamp-meter current readings and inverter diagnostic values captured during compressor ramp-up..
In writingThe peak current reading in amperes, the service manual test step results for U2-31, and the exact component diagnosis before quoting any PCB or compressor repair..
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 U2-31 help screen and malfunction history on the indoor user interface
- Who:
- You or an advanced end user at the indoor MMI controller.
- How:
- Go to menu [11] Malfunctioning, tap on the U2-31 error message to view the on-screen help text, and with advanced end user permission open [11] Malfunction history to inspect recent timestamps.
- Result that matters:
- Whether U2-31 appeared once as an isolated automatic-reset event or repeats across multiple operating cycles.
- If the malfunction history shows a single U2-31 entry and the heat pump continues heating normally, that points to a one-off grid transient rather than a standing hardware failure.
- If U2-31 logs repeatedly in the malfunction history or appears alongside other U2 supply voltage codes, that indicates an ongoing electrical supply or inverter load issue requiring technician measurement.
- Evidence to keep:
- A photo of the [11] Malfunctioning help screen and the dated list of U2-31 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
Verify at the consumer board that the heat pump circuit is not shared with other appliances
- Who:
- You, by reading the circuit directory at the household distribution panel without opening any enclosure.
- How:
- Inspect the labels on the household breaker panel and note whether any heavy domestic load started at the exact minute U2-31 was logged, without switching off the heat pump circuit breakers.
- Result that matters:
- Whether the heat pump supply circuits are dedicated or shared with other building equipment, and whether all breakers remain ON.
- If another appliance shares the same circuit or trips a breaker during compressor startup, that confirms an installation supply issue violating the dedicated power circuit requirement.
- If the heat pump runs on its own dedicated breakers and no household switching event coincided with the timestamp, that rules out a shared domestic branch circuit.
- Evidence to keep:
- A photo of the distribution board labels and a written note of any appliances operating when U2-31 occurred.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Inspect the exterior cable route and airflow perimeter around the outdoor unit
- Who:
- You, standing outside next to the outdoor unit without removing any casing panel.
- How:
- Look along the visible power supply conduit and outdoor casing for mechanical damage, spliced cable extensions, or debris obstructing the unit perimeter.
- Result that matters:
- Clean exterior surroundings and intact external cable entry with no visible splices or physical impact.
- If the external cable shows an exposed joint, crushed conduit, or impact damage, that points to an external wiring fault that an electrician must rectify before further load testing.
- If the exterior cabling and outdoor unit perimeter are clean and undamaged while U2-31 continues to recur, that isolates the investigation to internal switch box terminals and inverter current diagnostics.
- Evidence to keep:
- Photos of the outdoor unit identification label, exterior cable entry, and surrounding clearance.
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 U2-31
- code erreur Daikin Altherma U2-31
- codice U2-31 pompa di calore Daikin
- código U2-31 bomba de calor Daikin
- Daikin Altherma error U2-31
- Daikin Altherma Fehler U2-31
- Daikin heat pump code U2-31
- Daikin U2-31
- Daikin Wärmepumpe Fehlercode U2-31
- défaut U2-31 pompe à chaleur Daikin
- error Daikin Altherma U2-31
- errore Daikin Altherma U2-31
What it means
In the Daikin Altherma 4 H F installer reference guide, error code U2-31 is classified under Supply voltage error with the trigger instantaneous overcurrent, automatic reset, and continuous unit operation. Unlike permanent lockout faults in the U2 group that halt the heat pump until a manual reset, U2-31 logs a brief current surge on the outdoor inverter or mains supply path that cleared fast enough for heating or hot water production to continue. Because an overcurrent spike can stem from a grid transient, a shared circuit load, a loose switch box terminal, or an inverter stage anomaly, the code alone never proves that a board or compressor is faulty.
Warnings and stop conditions
- Highsource verified
Do not open the outdoor or indoor unit casing or switch box yourself: high-voltage capacitors require more than 10 minutes after mains disconnection to drop below 50 V DC, and opening the casing can disturb the R290 refrigerant leak detection 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
After commissioning, do not turn off the circuit breakers supplying the heat pump units during normal use, because the R290 leak detection sensor and freeze protection functions require continuous electrical power.
Evidence: Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Symptoms and causes
Probable causes · in order
- Mains voltage dip or switching transient on the outdoor power inputCommon
A brief supply voltage dip, phase imbalance, or switching transient on the property mains caused an instantaneous overcurrent peak on the outdoor unit power input.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Shared feeder circuit instead of a dedicated heat pump supplyPossible
Another high-draw appliance is connected to the same feeder instead of a dedicated power circuit, creating momentary current spikes when both loads start together.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose terminal or spliced power cable in the switch boxPossible
A loose terminal, spliced power cable, or deteriorating electrical connection inside the outdoor or indoor switch box generated a transient current surge under load.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Outdoor inverter board A1P or compressor drive transient faultLess common
An intermittent insulation breakdown or component defect in the outdoor inverter assembly A1P or compressor drive circuit triggered a momentary overcurrent protection pulse.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-31 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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