- Heat pumps
- Air-to-water heat pump
U1-01
Daikin Altherma 4 H F U1-01: reverse or open phase stop
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
U1-01 means the inverter PCB on a Daikin Altherma 4 H F detected a reverse phase or open phase condition, stopping the heat pump and requiring a manual reset. Check whether your building had a power outage or electrical work and record [11] Malfunction history before calling your installer.
Safety first
Stop and check this first
- Do not attempt to open the switch box or touch internal terminals: Daikin requires disconnecting all power supplies for more than 10 minutes and verifying that capacitor voltage is less than 50 V DC before touching electrical components.
- Never bypass phase monitoring or force repeated manual resets on U1-01; Daikin states that running the product in reversed phase can break the equipment and that the problem must be understood and corrected before resetting an error code.
Safe checks you can perform
- Read the long U1-01 description and log entries in [11] Malfunction history
- Check the building distribution board and note any recent electrical or utility work
- Verify that U1-01 requires a manual reset only after the electrical cause is understood
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure three-phase line-to-line voltages and verify phase sequence at the supply terminal block
- Do not attempt this professional task: Inspect the inverter PCB supply harness and test residual DC bus voltage before touching board terminals
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.
Measure three-phase line-to-line voltages and verify phase sequence at the supply terminal block
What they doA licensed electrician or authorised Daikin technician must verify that the power supply voltage matches the voltage on the identification label of the unit, measure all three line-to-line voltages under load, and test phase rotation with a phase-sequence meter.
They must show youlive line-to-line voltage readings across L1-L2, L2-L3, and L1-L3 plus the phase-rotation meter indication compared against the unit identification label.
In writingmeasured voltage values for each phase pair, the verified phase sequence, and any corrective action taken on the supply fuses or phase wiring.
Inspect the inverter PCB supply harness and test residual DC bus voltage before touching board terminals
What they doBefore touching electrical parts inside the switch box, the technician must disconnect the power supply for more than 10 minutes, confirm the residual voltage is less than 50 V DC at the measurement points shown on the wiring diagram label, and inspect the harness between the terminal block and inverter PCB.
They must show youthe DC voltage measurement below 50 V DC before disengaging connectors and the physical condition of the phase terminals feeding the inverter PCB.
In writingthe recorded post-discharge DC voltage reading, terminal torque and harness continuity results, and the manual reset verification after 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 long U1-01 description and log entries in [11] Malfunction history
- Who:
- The homeowner or system operator at the indoor user interface.
- How:
- Select the error banner on the home screen to read the long description for U1-01, then set user permission level to advanced end user and open [11]: Malfunction history.
- Result that matters:
- Whether U1-01 appeared as a single event or alongside other power-supply codes such as U1-00.
- If [11] Malfunction history shows U1-01 right after a utility outage or alongside U1-00, that points to an external supply disturbance or phase drop rather than an internal board failure.
- If U1-01 latches immediately every time the heat pump attempts to start with stable building power, that indicates a persistent open phase, reversed phase sequence, or internal wiring fault.
- Evidence to keep:
- A clear photo of the U1-01 error screen and the timestamped list in [11] Malfunction history.
Perform this check from the indoor user interface only; do not open the outdoor or indoor switch box.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check the building distribution board and note any recent electrical or utility work
- Who:
- The homeowner or facility manager inspecting the main consumer panel from the outside.
- How:
- Visually check whether all poles of the dedicated heat pump circuit breaker and earth leakage protector are in the ON position and note whether utility work, generator changeover, or a momentary blackout occurred before U1-01 appeared.
- Result that matters:
- Whether a breaker pole has tripped or recent electrical work coincided with the U1-01 lockout.
- If a breaker tripped or electrical work preceded the stop, that confirms an external supply interruption or altered phase rotation is the primary suspect.
- If all external breakers remain closed and no utility switching occurred, that rules out a visible consumer-panel trip and requires phase voltage and rotation measurements by an electrician.
- Evidence to keep:
- A photo of the dedicated breaker states in the consumer panel and written notes on any recent power outage or electrical work.
Never open the distribution board cover or attempt to reset a breaker that trips repeatedly.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Verify that U1-01 requires a manual reset only after the electrical cause is understood
- Who:
- The homeowner reviewing the user interface status before contacting the installer.
- How:
- Check whether the heat pump remains stopped with U1-01 active on the user interface and open [6.2]: Information > Dealer information to retrieve the installer contact number without forcing repeated resets.
- Result that matters:
- The unit remains stopped under U1-01 manual reset protection until the three-phase supply is verified.
- If the unit stays stopped with U1-01 displayed, that confirms the manual-reset protection is holding the compressor off as designed in the troubleshooting table.
- If anyone suggests resetting or power-cycling the unit repeatedly to force compressor operation without checking phase rotation, that points to a risk of equipment damage because Daikin requires understanding the problem before resetting.
- Evidence to keep:
- The model designations from the unit nameplates and the installer phone number shown in [6.2] Dealer information.
Do not force repeated manual resets or power cycles on U1-01 before phase presence and phase rotation have been verified.
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 U1-01
- code erreur Daikin Altherma U1-01
- codice U1-01 pompa di calore Daikin
- código U1-01 bomba de calor Daikin
- Daikin Altherma error U1-01
- Daikin Altherma Fehler U1-01
- Daikin heat pump code U1-01
- Daikin U1-01
- Daikin Wärmepumpe Fehlercode U1-01
- défaut U1-01 pompe à chaleur Daikin
- error Daikin Altherma U1-01
- errore Daikin Altherma U1-01
What it means
In the Daikin Altherma 4 H F installer reference guide (4PEN773391-1C), U1-01 is classified as Malfunction by reverse phase/ open-phase when an open phase or reverse phase condition is detected by the inverter PCB. Unlike U1-00, which resets automatically, U1-01 stops heat pump operation and requires a manual reset. The code alone does not prove that the inverter PCB has failed, because a dropped three-phase supply conductor, a blown supply fuse, a loose field terminal, or swapped phase sequence after utility work or a momentary blackout can trigger the same detection.
Warnings and stop conditions
- Highsource verified
Do not attempt to open the switch box or touch internal terminals: Daikin requires disconnecting all power supplies for more than 10 minutes and verifying that capacitor voltage is less than 50 V DC before touching electrical components.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Highsource verified
Never bypass phase monitoring or force repeated manual resets on U1-01; Daikin states that running the product in reversed phase can break the equipment and that the problem must be understood and corrected before resetting an error code.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Open phase or dropped conductor on the three-phase supply feeding the inverter PCBCommon
The inverter PCB monitors incoming three-phase supply lines and registers U1-01 when one phase is missing due to a blown fuse, tripped single-pole device, or loose supply terminal.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Reversed phase rotation after electrical work, commissioning, or a momentary blackoutCommon
Daikin warns that reversed phase sequence can occur while the power is turned on and off or after a momentary blackout, and the inverter PCB stops the heat pump when reverse rotation is detected.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose or degraded field wiring connection between the power supply terminal block and the inverter PCBPossible
High contact resistance or a partially unseated conductor inside the switch box can cause an intermittent open-phase reading at the inverter PCB under load.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Inverter PCB phase-detection circuit fault inside the heat pump unitLess common
If all three incoming phases show balanced line-to-line voltages matching the identification label and correct clockwise phase rotation right at the board input, the inverter PCB itself can be faulty.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U1-01 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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