- Heat pumps
- Air-to-water heat pump
U2-03
Daikin Altherma 4 H F U2-03: PN short-circuit voltage error
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Daikin Altherma 4 H F error U2-03 indicates a supply voltage error with the trigger PN short-circuit error, which stops the heat pump and requires a manual reset. Avoid resetting the unit or switching off breakers; record the code from [11] Malfunctioning and call Daikin service.
Safety first
Stop and check this first
- Do not open the unit (especially the outdoor unit) or attempt internal repairs for U2-03: the outdoor unit contains R290 flammable refrigerant and high-voltage inverter components that only a Daikin certified technician may service.
- When carrying out an inspection on the switch box of the unit, the technician must always make sure that the unit is disconnected from the mains by turning off the respective circuit breaker.
Safe checks you can perform
- Read the U2-03 error details and check [11] Malfunction history
- Leave manual reset and circuit breakers untouched before inspection
- Check [5.23] Emergency selection for backup heater operation while waiting
Leave these tasks to a qualified professional
- Do not attempt this professional task: Isolate mains power and diagnose the PN circuit per the Daikin service manual
- Do not attempt this professional task: Replace damaged inverter or wiring components and verify 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.
Isolate mains power and diagnose the PN circuit per the Daikin service manual
What they doA Daikin certified technician must follow the R290 safety checklist, disconnect the unit from the mains at the circuit breaker, inspect the outdoor unit switch box and internal wiring, and test the inverter PCB and PN bus components against the service manual.
They must show youthe component test readings from the service manual procedure that isolate the PN short-circuit source.
In writingthe measured electrical values, the defective board or wiring part number identified, and the service manual step completed for U2-03.
Replace damaged inverter or wiring components and verify manual reset
What they doIf internal wiring or the inverter PCB is damaged, the manufacturer, service agent, or similarly qualified person must replace the faulty part, restore power, perform the manual reset for U2-03, and run an operation test.
They must show youthe replaced component and the [11] Malfunctioning screen showing no active U2-03 after a test run.
In writingthe replacement part reference, post-repair test duration, and confirmation that U2-03 did not recur.
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 U2-03 error details and check [11] Malfunction history
- Who:
- You, at the indoor unit user interface touchscreen.
- How:
- Open [11] Malfunctioning, tap U2-03 to display the long help text, and if advanced end user permission is active, open [11] Malfunction history.
- Result that matters:
- Knowing when U2-03 occurred and whether earlier supply voltage or inverter codes preceded it.
- If U2-03 appears repeatedly in Malfunction history after a previous reset, that confirms a persistent fault inside the outdoor unit power circuit.
- If U2-03 is a first-time entry with no prior errors, that indicates a newly triggered PN short-circuit condition that still requires professional inspection before resetting.
- Evidence to keep:
- Take a photo of the U2-03 long description and the [11] Malfunction history screen.
Never open the outdoor unit cover to look for damaged components; the unit contains R290 refrigerant and high-voltage inverter circuits.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Leave manual reset and circuit breakers untouched before inspection
- Who:
- You (the end user), without touching the electrical panel or resetting the error.
- How:
- Keep the unit circuit breakers in their normal ON state and refrain from clearing U2-03 on the controller before a technician inspects the outdoor unit.
- Result that matters:
- Preventing repeated energization of a potential PN short-circuit while keeping R290 leak detection powered.
- If a manual reset was already attempted and U2-03 returned immediately, that confirms an active PN short-circuit condition and rules out a one-off glitch.
- If no reset has been attempted yet, that preserves the fault state for the technician to diagnose safely according to Daikin troubleshooting rules.
- Evidence to keep:
- Note whether any reset was attempted and the exact time U2-03 reappeared if applicable.
Do not turn off the circuit breakers to the units after commissioning, and never reset a safety device before finding why it activated.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check [5.23] Emergency selection for backup heater operation while waiting
- Who:
- You, from the indoor unit Settings menu.
- How:
- Go to [5.23] Settings > Emergency selection (or respond to the [5.30] Emergency acknowledgement pop-up if shown) to see whether the electrical backup heater or booster heater can take over space heating and domestic hot water.
- Result that matters:
- Determining whether indoor heating and hot water remain available while the outdoor heat pump is stopped by U2-03.
- If Emergency selection is set to Automatic or you confirm the [5.30] pop-up, that confirms the electrical heater can take over space heating and DHW operation at higher electricity consumption.
- If the backup heater does not take over or its circuit breaker F1B has tripped, that indicates the house has no backup heat and increases the urgency of the service visit.
- Evidence to keep:
- Save a photo of the [5.23] Emergency selection setting and note whether backup heating is active.
Be aware that when the backup heater or booster heater takes over the heat load during a heat pump failure, electricity consumption is considerably higher.
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 U2-03
- code erreur Daikin Altherma U2-03
- codice U2-03 pompa di calore Daikin
- código U2-03 bomba de calor Daikin
- Daikin Altherma error U2-03
- Daikin Altherma Fehler U2-03
- Daikin heat pump code U2-03
- Daikin U2-03
- Daikin Wärmepumpe Fehlercode U2-03
- défaut U2-03 pompe à chaleur Daikin
- error Daikin Altherma U2-03
- errore Daikin Altherma U2-03
What it means
In Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 214), U2-03 is listed as a Supply voltage error triggered by a PN short-circuit error. Upon detection, the outdoor unit controller halts heat pump operation and locks out until a manual reset is performed. Because a PN short-circuit detection in the inverter DC bus path requires internal electrical testing of the outdoor unit power circuit and inverter PCB using the Daikin service manual, the error code alone does not identify which specific component failed.
Warnings and stop conditions
- Highsource verified
Do not open the unit (especially the outdoor unit) or attempt internal repairs for U2-03: the outdoor unit contains R290 flammable refrigerant and high-voltage inverter components that only a Daikin certified technician may service.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Highsource verified
When carrying out an inspection on the switch box of the unit, the technician must always make sure that the unit is disconnected from the mains by turning off the respective circuit breaker.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Short-circuit fault on the outdoor unit inverter PN power circuitCommon
An insulation breakdown or component failure on the inverter PCB or connected outdoor unit power stage triggers the PN short-circuit error and stops the heat pump.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Damaged internal wiring or loose connection inside the outdoor unit switch boxPossible
Damaged internal conductors or defective wiring inside the outdoor unit switch box causes the inverter protection circuit to register a PN short-circuit condition.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Transient electrical surge triggering PN short-circuit protection on U2-03Less common
A severe power line disturbance trips the PN short-circuit detection once without permanent component damage, which can only be confirmed after a technician inspects the unit.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-03 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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