- Heat pumps
- Air-to-water heat pump
U2-01
Daikin Altherma 4 H F U2-01: inverter supply voltage error
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Daikin Altherma 4 H F units, U2-01 is a supply voltage error triggered by inverter undervoltage or overvoltage that stops heat pump operation and resets automatically. Note any household power dips or flickering lights, leave the circuit breakers ON, and call your installer if U2-01 recurs.
Safety first
Stop and check this first
- Never open the outdoor unit or switch box yourself and do not turn off the unit circuit breakers to clear U2-01: the outdoor unit contains flammable R290 refrigerant and relies on continuous power to run its gas leak detection sensor and dilution fan.
- When a safety device such as a fuse or circuit breaker activates alongside U2-01, stop the unit and have a qualified person find the cause before resetting it; never shunt safety devices.
Safe checks you can perform
- Correlate U2-01 with household power disturbances
- Visually inspect the heat pump circuit breakers without switching them off
- Count U2-01 recurrences in [11] Malfunction history
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure live outdoor unit supply voltage against the identification label
- Do not attempt this professional task: Inspect switch box terminals, cable continuity, and inverter branch per service manual
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 live outdoor unit supply voltage against the identification label
What they doHave a qualified installer or authorised electrician measure the voltage at the outdoor unit supply terminals at rest and under compressor load, comparing readings against the identification label (220-240 V for 1~ V3 models or 380-415 V for 3N~ W1 models).
They must show youthe multimeter voltage readings at the outdoor unit supply terminals compared with the identification label rating.
In writingthe measured resting and loaded voltages per phase, measurement timestamp, and whether utility supply is within specification for U2-01.
Inspect switch box terminals, cable continuity, and inverter branch per service manual
What they doWith mains power isolated at the circuit breaker and R290 safety precautions observed, the technician must inspect terminal block X1M for loose connections or spliced cables, verify tightening torque (2.45 N*m on X1M M5), and follow the Daikin service manual if U2-01 persists with normal supply voltage.
They must show youthe switch box terminal inspection results and the post-test [11] Malfunction history free of new U2-01 entries.
In writingthe terminal torque check, fuse and RCD ratings verified, and any part reference replaced under U2-01 diagnosis.
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.
Correlate U2-01 with household power disturbances
- Who:
- You (the homeowner or end user) can do this check without tools.
- How:
- Check whether lights flickered, appliances rebooted, or a utility outage occurred at the exact time U2-01 appeared on the indoor unit user interface.
- Result that matters:
- Whether the inverter undervoltage or overvoltage coincided with a wider domestic mains disturbance.
- If other household circuits experienced a simultaneous dip or outage, that points to an external utility grid event where automatic reset should restore normal heat pump operation.
- If U2-01 appears repeatedly while all other household electrical equipment runs normally, that rules out a general blackout and points to the dedicated heat pump supply circuit or outdoor unit.
- Evidence to keep:
- Write down the date, time, weather conditions, and any observed utility power disturbances.
Never open the outdoor or indoor unit switch box to check voltage yourself; live mains terminals carry an electrocution risk.
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 heat pump circuit breakers without switching them off
- Who:
- You can perform this visual check at the household electrical cabinet.
- How:
- Look through the electrical cabinet window to verify whether the circuit breakers marked with 'Do NOT turn OFF the circuit breaker' stickers (one breaker for normal kWh rate supply, or two for preferential kWh rate supply) are in the ON position.
- Result that matters:
- Whether all dedicated heat pump protection devices remain engaged while U2-01 is active or recovering.
- If a circuit breaker or residual current device has tripped OFF, that confirms a protection event occurred and indicates you must leave it off until the installer finds why the safety device activated.
- If every breaker is still ON, that confirms power reaches the line and allows the controller to execute its automatic reset for U2-01 without manual power cycling.
- Evidence to keep:
- Take a clear photo of the electrical cabinet showing the breaker lever positions and reference stickers.
After commissioning, do not turn off the circuit breakers to the units so that R290 leak detection and anti-blockage protections remain activated.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Count U2-01 recurrences in [11] Malfunction history
- Who:
- You, from the indoor unit touchscreen with advanced end user permission enabled.
- How:
- Open [11] Malfunctioning to read the long description of U2-01, then go to [11] Malfunction history to see how many times U2-01 has been logged recently.
- Result that matters:
- Whether U2-01 was a single isolated event that cleared automatically or a recurring supply voltage fault.
- If U2-01 logged only once during a known utility dip and the status indicator returns to solid or pulsing blue, that confirms the automatic reset succeeded.
- If U2-01 reappears multiple times in Malfunction history, that indicates a persistent supply voltage or inverter condition requiring an installer visit via [6.2] Dealer information.
- Evidence to keep:
- Save photos of the U2-01 long description screen and the timestamped list in [11] Malfunction history.
Do not enter installer Maintenance mode [7] while checking error logs, as doing so disables automatic protective functions for up to 12 hours.
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 U2-01
- code erreur Daikin Altherma U2-01
- codice U2-01 pompa di calore Daikin
- código U2-01 bomba de calor Daikin
- Daikin Altherma error U2-01
- Daikin Altherma Fehler U2-01
- Daikin heat pump code U2-01
- Daikin U2-01
- Daikin Wärmepumpe Fehlercode U2-01
- défaut U2-01 pompe à chaleur Daikin
- error Daikin Altherma U2-01
- errore Daikin Altherma U2-01
What it means
In Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 214), code U2-01 is classified under Supply voltage error with the trigger Inverter undervoltage/ overvoltage. When the outdoor unit inverter controller detects voltage outside its operating window, the heat pump stops operating and performs an automatic reset once supply voltage returns to normal. Because external utility voltage fluctuations, mismatched supply wiring, loose switch box terminals, or inverter PCB sensing issues can all trigger this threshold, code U2-01 alone never proves that the inverter board or any internal component is defective.
Warnings and stop conditions
- Highsource verified
Never open the outdoor unit or switch box yourself and do not turn off the unit circuit breakers to clear U2-01: the outdoor unit contains flammable R290 refrigerant and relies on continuous power to run its gas leak detection sensor and dilution fan.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Mediumsource verified
When a safety device such as a fuse or circuit breaker activates alongside U2-01, stop the unit and have a qualified person find the cause before resetting it; never shunt safety devices.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Temporary utility grid undervoltage or overvoltage event on U2-01Common
A brief dip, brownout, or swell on the domestic mains supply pushes the inverter voltage outside limits and triggers U2-01, which then clears automatically when grid voltage stabilizes.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Outdoor unit supply voltage not matching the identification labelPossible
If the incoming supply voltage does not match the unit identification label (220-240 V on 1~ V3 models or 380-415 V on 3N~ W1 models) or drops under load, the inverter logs repeated U2-01 stops.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose power terminals or spliced supply cable causing voltage dropPossible
Loose connections inside the switch box or prohibited cable extensions using wire connectors, clamps, or taped wires introduce resistance and voltage fluctuations at the outdoor unit inverter.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-01 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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