- Heat pumps
- Air-to-water heat pump
U2-02
Daikin Altherma 4 H F U2-02: open-phase power supply error
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Code U2-02 on a Daikin Altherma 4 H F means a supply voltage error caused by an open-phase of the power supply (undervoltage or overvoltage during current-limiting operation), stopping the heat pump with a manual reset. Check your breaker panel visually without touching it and call your installer.
Safety first
Stop and check this first
- Never inspect the switch box while energized or bridge a blown fuse: all wiring and open-phase repairs must be performed by an authorised electrician with the circuit breaker turned off.
- If the power supply has a missing or wrong N-phase, the heat pump equipment can break down; do not repeatedly reset U2-02 without measuring all supply conductors.
Safe checks you can perform
- Look for a tripped circuit breaker or fuse indicator in the distribution panel
- Check whether other three-phase or household appliances lost power
- Capture the U2-02 details in [11] Malfunctioning and dealer contact in [6.2]
Leave these tasks to a qualified professional
- Do not attempt this professional task: Verify phase-to-neutral and phase-to-phase voltages and N-phase continuity
- Do not attempt this professional task: Inspect field fuses, RCDs, and X1M terminals before executing 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.
Verify phase-to-neutral and phase-to-phase voltages and N-phase continuity
What they doAn authorised electrician or Daikin installer must measure each supply phase and neutral conductor at the local supply panel and at outdoor unit terminal block X1M, confirming there is no open phase or missing/wrong N-phase.
They must show youthe measured voltage on every phase and neutral terminal at X1M compared against the identification label.
In writingthe phase voltage readings, neutral continuity result, and the exact location of any open-phase fault found for U2-02.
Inspect field fuses, RCDs, and X1M terminals before executing manual reset
What they doAfter disconnecting the unit from the mains at the circuit breaker, the technician must verify that field fuses (F1U, 16 A or 25 A C curve as specified) and the 30 mA RCD (Q1DI) match Daikin specifications, check X1M wire retainers and torque, and then perform the manual reset and test run.
They must show youthe inspected X1M terminal connections, protection device ratings, and the user interface running without U2-02.
In writingthe fuse and breaker specifications checked, corrective wiring actions taken, and manual reset test result.
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.
Look for a tripped circuit breaker or fuse indicator in the distribution panel
- Who:
- You (the property owner or user), by visual inspection only.
- How:
- Inspect the domestic electrical cabinet without opening its cover or flipping levers to see if the outdoor unit fuse/breaker (F1U) or earth leakage circuit breaker (Q1DI) has tripped.
- Result that matters:
- Whether a field protection device on the heat pump supply line has interrupted a conductor.
- If a breaker or protection device is in the tripped position, that confirms an electrical trip on the supply line and indicates you must wait for an electrician or installer to find the cause before resetting it.
- If all breakers remain in their normal ON position, that rules out a visible panel trip and points to a utility phase loss, internal switch box connection, or outdoor unit sensing fault.
- Evidence to keep:
- Photograph the distribution board showing all heat pump breaker positions and labels.
Never reset a tripped safety device or open the electrical panel yourself; Daikin requires finding the cause of activation before resetting.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check whether other three-phase or household appliances lost power
- Who:
- You, by checking other rooms and appliances in the building.
- How:
- Verify whether other circuits, rooms, or three-phase loads in the house lost power or dimmed when U2-02 stopped the heat pump.
- Result that matters:
- Whether the open-phase condition is affecting the entire building supply or only the Daikin heat pump branch.
- If part of the house or other multi-phase equipment also lost power, that points to an upstream utility supply phase outage rather than a fault inside the heat pump.
- If every other household circuit works normally, that isolates the open-phase condition to the dedicated heat pump supply cable, field fuse, or outdoor unit switch box.
- Evidence to keep:
- Record the time of the U2-02 stop and list any other household circuits or appliances affected.
Do not attempt to test phase voltages with a multimeter at the unit or panel; live mains work must be performed by an authorised electrician.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Capture the U2-02 details in [11] Malfunctioning and dealer contact in [6.2]
- Who:
- You, using the indoor unit touchscreen display.
- How:
- Navigate to [11] Malfunctioning, tap the U2-02 message to view the full help text, and open [6.2] Information > Dealer information for the installer contact number.
- Result that matters:
- Having the exact error code, timestamp, and long description ready before any manual reset is attempted.
- If additional U1 or U2 codes appear alongside U2-02 in the [11] Malfunctioning list, that confirms multiple supply or phase detections occurred on the outdoor unit.
- If U2-02 is the sole active code, that isolates the event to open-phase undervoltage or overvoltage during current-limiting operation for the technician to verify.
- Evidence to keep:
- Keep a photo of the [11] Malfunctioning screen showing U2-02, its date and time, and the long description text.
Do not clear U2-02 by cycling the main circuit breakers; 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
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 U2-02
- code erreur Daikin Altherma U2-02
- codice U2-02 pompa di calore Daikin
- código U2-02 bomba de calor Daikin
- Daikin Altherma error U2-02
- Daikin Altherma Fehler U2-02
- Daikin heat pump code U2-02
- Daikin U2-02
- Daikin Wärmepumpe Fehlercode U2-02
- défaut U2-02 pompe à chaleur Daikin
- error Daikin Altherma U2-02
- errore Daikin Altherma U2-02
What it means
According to Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 214), error U2-02 is a Supply voltage error whose documented trigger is Open-phase of power supply (undervoltage/ overvoltage during current-limiting operation). When this condition is detected, the heat pump stops operating and requires a manual reset rather than clearing on its own. An open phase or neutral imbalance can originate in the utility grid, a tripped pole or fuse in the field supply panel, a loose conductor at terminal block X1M, or the outdoor unit power electronics, so U2-02 alone does not prove the outdoor unit is faulty.
Warnings and stop conditions
- Highsource verified
Never inspect the switch box while energized or bridge a blown fuse: all wiring and open-phase repairs must be performed by an authorised electrician with the circuit breaker turned off.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Mediumsource verified
If the power supply has a missing or wrong N-phase, the heat pump equipment can break down; do not repeatedly reset U2-02 without measuring all supply conductors.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Open phase or missing neutral on the field power supply lineCommon
A blown field fuse (F1U), tripped pole, or interrupted conductor in the supply line (3N~ 380-415 V on W1 models or 1~ 220-240 V on V3 models) causes undervoltage or overvoltage during current-limiting operation.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose wire termination in the outdoor unit switch box or supply panelPossible
An improperly clamped conductor or loose screw on outdoor unit terminal block X1M creates an intermittent open-phase condition when current demand rises.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Partial phase loss or severe voltage imbalance from the electricity gridPossible
A utility supply fault affecting one phase of the building feed triggers U2-02 while the heat pump operates under current limitation.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-02 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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