- Heat pumps
- Air-to-water heat pump
U2-43
Daikin Altherma U2-43 supply error: overvoltage in operation
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma 4 H F triggers U2-43 when it detects an overvoltage during operation under the supply voltage error category, stopping the heat pump until a manual reset. Open menu [11] Malfunctioning to record the error details and ask an electrician to measure your mains supply voltage.
Safety first
Stop and check this first
- Never open the switch box or unit panels yourself: always disconnect mains power for more than 10 minutes and measure less than 50 V DC before touching internal electrical parts, and never disturb the R290 leak sensor inside the casing.
- If the power supply has a missing or wrong N-phase, severe overvoltage can destroy electrical equipment; never reset U2-43 or shunt safety devices until a qualified technician has verified the neutral and supply voltage.
Safe checks you can perform
- Inspect the U2-43 message in menu [11] Malfunctioning and check timestamps in [11] Malfunction history
- Check whether a site solar inverter, voltage monitor, or other appliance reported high grid voltage
- Verify emergency heating status in [5.23] without switching off the unit circuit breakers
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure live supply voltage, verify N-phase integrity, and inspect the switch box terminals
- Do not attempt this professional task: Follow the service manual procedure for U2-43 to test the outdoor PCB A1P before 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.
Measure live supply voltage, verify N-phase integrity, and inspect the switch box terminals
What they doA qualified electrician or Daikin technician must measure phase-to-neutral and phase-to-phase voltages against the unit identification label (220-240 V or 380-415 V, 50 Hz), then isolate the mains for more than 10 minutes until capacitor voltage is below 50 V DC to inspect the switch box for loose terminals or a missing/wrong N-phase.
They must show youThe multimeter readings of each phase-to-neutral voltage and neutral continuity, plus the switch box terminal inspection after the 10-minute 50 V DC discharge check..
In writingAll measured L-N and L-L voltages under standby and load, verification of intact N-phase wiring, and confirmation of dedicated circuit compliance on the service report..
Follow the service manual procedure for U2-43 to test the outdoor PCB A1P before manual reset
What they doIf the incoming mains supply and neutral connection remain stable within specification during operation, the Daikin technician must use the model service manual to check the outdoor main PCB A1P voltage detection and inverter bus circuit before performing a manual reset.
They must show youThe service manual diagnostic readings for the outdoor PCB voltage sensor compared with the actual multimeter reading on the supply terminals..
In writingThe comparison between actual mains voltage and PCB-monitored voltage, the service manual test outcome for U2-43, and written confirmation that the unit completed a load test after manual reset..
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.
Inspect the U2-43 message in menu [11] Malfunctioning and check timestamps in [11] Malfunction history
- Who:
- You or an advanced end user checking the indoor unit display.
- How:
- Go to [11] Malfunctioning, tap on the U2-43 error screen to view the help text, and open [11] Malfunction history to note the exact time of day each U2-43 event occurred.
- Result that matters:
- The timestamps of U2-43 and whether any other U2 supply codes (such as U2-01 or U2-44) appear in the history.
- If U2-43 consistently occurs during peak midday solar generation or late-night low grid load, that points to an external utility or site overvoltage condition during operation.
- If U2-43 triggers immediately whenever the heat pump starts regardless of time of day, that indicates a permanent wiring fault (such as a loose neutral) or an internal board sensing error.
- Evidence to keep:
- Photos of the [11] Malfunctioning help screen and the dated log 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
Check whether a site solar inverter, voltage monitor, or other appliance reported high grid voltage
- Who:
- You, checking the display or app of any household solar PV inverter, smart meter, or UPS without touching electrical wiring.
- How:
- Compare the timestamp of the U2-43 lockout against your solar inverter log or utility monitor to see if site voltage exceeded the nominal 220-240 V single-phase or 380-415 V three-phase range.
- Result that matters:
- Whether external monitoring also recorded high mains voltage or grid overvoltage trips at the same time.
- If your solar inverter or smart meter logged a high-voltage event at the same timestamp, that confirms the heat pump reacted to an external supply overvoltage rather than an internal defect.
- If site voltage remained well within the nominal range while only the heat pump stopped with U2-43, that isolates the fault to the heat pump supply circuit, neutral connection, or outdoor PCB.
- Evidence to keep:
- A screenshot or photo of any site voltage reading or solar inverter event log matching the U2-43 timestamp.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Verify emergency heating status in [5.23] without switching off the unit circuit breakers
- Who:
- You, using the indoor MMI menu while waiting for qualified electrical inspection.
- How:
- Leave the unit circuit breakers ON so the R290 leak sensor stays powered, check [5.23] Settings > Emergency selection to see if backup heating is enabled while the heat pump is stopped, and open [6.2] Information > Dealer information.
- Result that matters:
- Whether emergency operation is available and the dealer contact details are on hand.
- If the indoor hydrobox operates cleanly on its supply without tripping breakers, that rules out a total building blackout and keeps the focus on the outdoor unit operational voltage.
- If other appliances in the building flicker brightly or fail at the same time, that points to a dangerous loose neutral on the property supply requiring immediate electrician intervention.
- Evidence to keep:
- A note of the [5.23] Emergency selection setting and any observed symptoms on other household circuits.
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 U2-43
- code erreur Daikin Altherma U2-43
- codice U2-43 pompa di calore Daikin
- código U2-43 bomba de calor Daikin
- Daikin Altherma error U2-43
- Daikin Altherma Fehler U2-43
- Daikin heat pump code U2-43
- Daikin U2-43
- Daikin Wärmepumpe Fehlercode U2-43
- défaut U2-43 pompe à chaleur Daikin
- error Daikin Altherma U2-43
- errore Daikin Altherma U2-43
What it means
In the English Daikin Altherma 4 H F installer reference guide (the original instruction language), error code U2-43 is defined under Supply voltage error as Overvoltage during operation, stopping the heat pump until a manual reset. While U2-44 flags undervoltage during operation and U2-31 logs an instantaneous overcurrent, U2-43 indicates that the supply or inverter bus voltage rose above the operating ceiling while the heat pump was running. Because operational overvoltage can stem from high utility voltage, a missing or wrong N-phase on a three-phase supply, a loose switch box terminal, or an outdoor PCB sensing fault, the code alone does not prove a board failure.
Warnings and stop conditions
- Highsource verified
Never open the switch box or unit panels yourself: always disconnect mains power for more than 10 minutes and measure less than 50 V DC before touching internal electrical parts, and never disturb the R290 leak sensor inside the casing.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Highsource verified
If the power supply has a missing or wrong N-phase, severe overvoltage can destroy electrical equipment; never reset U2-43 or shunt safety devices until a qualified technician has verified the neutral and supply voltage.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- High utility grid voltage or site solar export swell during operationCommon
The incoming utility mains voltage rose above the rated supply range on the unit identification label (220-240 V on 1N~ models or 380-415 V on 3N~ models) during heat pump operation, such as during high local solar export or grid voltage swell.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Missing, loose, or wrong N-phase connection causing phase unbalanceCommon
A missing, loose, or wrong N-phase connection in the power supply or switch box caused phase-to-neutral voltage unbalance and drove one phase into overvoltage under load.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Loose terminal or defective wiring inside the switch boxPossible
A loose terminal, damaged conductor, or improperly spliced power cable in the outdoor switch box created intermittent contact and transient voltage spikes during compressor or fan operation.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Outdoor main PCB A1P voltage sensing or bus regulation faultLess common
The voltage monitoring or regeneration circuit on the outdoor main printed circuit board A1P misread normal line voltage or failed to regulate internal bus voltage during operation.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-43 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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