- Heat pumps
- Air-to-water heat pump
U1-00
Daikin Altherma U1-00: reverse phase or open-phase fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma U1-00 means the outdoor inverter PCB detected a reverse phase or open-phase condition on the power supply and stopped the heat pump. Check your distribution board for a tripped pole, record menu [11] Malfunctioning, and call an electrician to test the supply phases.
Safety first
Stop and check this first
- Never attempt to swap three-phase conductors or open the outdoor switch box yourself. Wait more than 10 minutes after disconnecting all power supplies and verify less than 50 V DC before a qualified professional touches internal electrical parts.
- Never force the equipment to run in reversed phase or bypass phase protection; Daikin explicitly warns that running the product in reversed phase can break the unit.
Safe checks you can perform
- Check the distribution board breakers and note any recent utility outages or electrical work
- Inspect menu [11] Malfunctioning and the malfunction history to distinguish U1-00 from U1-01 or U2-02
- Check model supply type on the nameplate and verify backup heating settings in [5.23]
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure phase-to-phase and phase-to-neutral voltages and verify clockwise phase rotation
- Do not attempt this professional task: Isolate power for over 10 minutes (<50 V DC) and inspect the outdoor switch box and inverter PCB
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 phase-to-phase and phase-to-neutral voltages and verify clockwise phase rotation
What they doA qualified electrician or Daikin technician must measure L1-L2, L2-L3, L3-L1 (400 V on 3N~ W1 models) and phase-to-neutral voltages (230 V), check phase rotation with a phase-sequence meter, and fit a reversed-phase protection circuit if momentary blackouts can reverse phase.
They must show youThe live multimeter voltage readings across all supply terminals and the phase-rotation meter indication.
In writingMeasured phase-to-phase and phase-to-neutral voltages (V AC), confirmed phase sequence (L1-L2-L3), and terminal torque check result.
Isolate power for over 10 minutes (<50 V DC) and inspect the outdoor switch box and inverter PCB
What they doBefore touching internal wiring or replacing the inverter PCB, the technician must turn off all power supplies, wait more than 10 minutes, confirm less than 50 V DC on the capacitors, and verify continuity from the outdoor terminal block to the inverter PCB.
They must show youThe sub-50 V DC capacitor discharge measurement and the condition of the outdoor switch box power terminals.
In writingResidual DC bus voltage reading (<50 V DC after 10 minutes), continuity test results, and confirmation that U1-00 clears on restart.
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.
Check the distribution board breakers and note any recent utility outages or electrical work
- Who:
- You, visually inspecting the household electrical panel without touching live wiring.
- How:
- Check whether the multi-pole circuit breaker feeding the outdoor unit has tripped, and note whether the utility grid recently flickered or an electrician worked on the property.
- Result that matters:
- Whether all poles of the outdoor unit breaker are engaged and whether U1-00 followed a grid event or wiring change.
- If U1-00 appeared immediately after meter work, panel servicing, or a momentary blackout, that strongly points to reversed phase rotation or a dropped utility phase.
- If no grid event occurred and the breaker trips again when reset once, that indicates an active supply fault that must be tested by an electrician.
- Evidence to keep:
- Record the exact time of the outage or breaker trip and take a photo of the distribution board switches.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Inspect menu [11] Malfunctioning and the malfunction history to distinguish U1-00 from U1-01 or U2-02
- Who:
- You, using the indoor user interface.
- How:
- Open [11] Malfunctioning to view the full fault text, and switch user permission to advanced end user to review the malfunction history log.
- Result that matters:
- Whether U1-00 is logged alone (automatic reset) or accompanied by U1-01 (manual reset) or U2-02 (supply voltage open-phase error).
- If U1-00 cleared automatically once stable utility power returned, that confirms a temporary grid phase drop rather than a permanent wiring error.
- If U1-00 remains active or alternates with U1-01 or U2-02, that confirms a standing open-phase or reverse-phase fault at the outdoor unit supply.
- Evidence to keep:
- Take a photo of the [11] Malfunctioning screen and the history log entries before calling your installer.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check model supply type on the nameplate and verify backup heating settings in [5.23]
- Who:
- You, reading the exterior nameplate and the indoor user interface.
- How:
- Read the exterior model label to see if your unit is a W1 (3N~ 400 V) or V3 (1N~ 230 V) model, and check [5.23] Settings > Emergency selection on the indoor interface.
- Result that matters:
- The exact model code and supply designation, plus whether emergency backup heat can run safely if its circuit is unaffected.
- If the unit is a 3N~ W1 model installed or modified recently, that points directly to three-phase rotation or a missing phase leg.
- If the outdoor unit remains locked out on U1-00 while indoor controller menus work normally, that isolates the fault to the outdoor power feed or outdoor inverter PCB.
- Evidence to keep:
- Note the full outdoor model code from the nameplate and the installer contact number 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 U1-00
- code erreur Daikin Altherma U1-00
- codice U1-00 pompa di calore Daikin
- código U1-00 bomba de calor Daikin
- Daikin Altherma error U1-00
- Daikin Altherma Fehler U1-00
- Daikin heat pump code U1-00
- Daikin U1-00
- Daikin Wärmepumpe Fehlercode U1-00
- défaut U1-00 pompe à chaleur Daikin
- error Daikin Altherma U1-00
- errore Daikin Altherma U1-00
What it means
In chapter 14.4.4 of the Daikin Altherma 4 H F installer reference guide (4PEN773391-1C), code U1-00 is titled Malfunction by reverse phase/ open-phase, triggered when an open phase or reverse phase is detected by the inverter PCB. Unlike U1-01 (which shares the same trigger but requires a manual reset), U1-00 stops the heat pump and resets automatically when normal phase supply returns. The code alone does not prove the inverter PCB is defective: a momentary grid blackout, swapped phase rotation after utility or electrical work on 3N~ W1 models, a loose terminal in the outdoor switch box, or an open conductor on one supply leg can all trigger U1-00.
Warnings and stop conditions
- Highsource verified
Never attempt to swap three-phase conductors or open the outdoor switch box yourself. Wait more than 10 minutes after disconnecting all power supplies and verify less than 50 V DC before a qualified professional touches internal electrical parts.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Highsource verified
Never force the equipment to run in reversed phase or bypass phase protection; Daikin explicitly warns that running the product in reversed phase can break the unit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Lost or open phase on the incoming power supply cableCommon
A blown utility fuse, a tripped pole on the distribution breaker, or a loose terminal on the power supply cable (such as the 3N~ W1 400 V supply or 1N~ V3 230 V supply) drops voltage on one phase leg.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Reversed phase sequence following electrical work or a momentary blackoutCommon
Phase conductors L1, L2 and L3 were connected in reverse rotation during installation, meter replacement, generator changeover, or after a momentary power outage.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose or oxidized mains terminal inside the outdoor unit switch boxPossible
High contact resistance at the outdoor terminal block or isolator switch causes an open-phase voltage drop as soon as the inverter draws starting current.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Inverter PCB phase-detection circuit faultLess common
If all three incoming phase voltages and phase rotation are verified normal at the outdoor inverter PCB input, the board's internal phase-sensing circuit is faulty.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U1-00 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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