- Heat pumps
- Air-to-water heat pump
U2-04
Daikin Altherma 4 H F U2-04: SP-PAM undervoltage confirmed
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Error U2-04 on a Daikin Altherma 4 H F is a supply voltage error meaning SP-PAM undervoltage confirmed, which stops the heat pump and requires a manual reset. Record the message in [11] Malfunctioning, check whether household lights dipped, and call your installer before resetting.
Safety first
Stop and check this first
- Do not open the outdoor unit or measure live terminals yourself to troubleshoot U2-04: the unit operates at dangerous mains and inverter voltages and contains R290 flammable refrigerant.
- Never extend the power supply or interconnection cable with wire connectors, clamps, taped wires, or extension cords, as these cause voltage drop, overheating, electric shock, or fire.
Safe checks you can perform
- Review U2-04 and active fault types in [11] Malfunctioning
- Check for brownouts or heavy electrical loads running at the time of U2-04
- Verify circuit breaker levers visually and check dealer details in [6.2]
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure supply voltage under load and inspect field cable installation
- Do not attempt this professional task: Follow Daikin service manual diagnostics for SP-PAM if supply voltage is normal
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 supply voltage under load and inspect field cable installation
What they doHave a qualified installer or electrician measure the voltage at outdoor unit terminal block X1M both on standby and during compressor start-up, verify that the voltage matches the identification label, and check that multicore supply cables have no splices or loose terminals.
They must show youthe multimeter voltage readings at X1M under load alongside the unit identification label voltage.
In writingthe standby and loaded voltage values, wire size and terminal check results, and whether line voltage drop caused U2-04.
Follow Daikin service manual diagnostics for SP-PAM if supply voltage is normal
What they doIf the measured supply voltage stays within 220-240 V (1~ V3) or 380-415 V (3N~ W1) and all terminals are tight, the technician must follow the Daikin service manual for U2-04 to test the outdoor unit power electronics before performing the manual reset.
They must show youthe service manual test results and the [11] Malfunction history after manual reset and test run.
In writingthe diagnostic measurements on the outdoor unit PCB, any replaced component reference, and confirmation of normal operation.
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.
Review U2-04 and active fault types in [11] Malfunctioning
- Who:
- You, on the indoor unit user interface touchscreen.
- How:
- Go to [11] Malfunctioning, tap U2-04 to read the long description, and use the Filter button if needed to see whether any other Technical, Protection, or Safety errors are listed.
- Result that matters:
- Identifying the exact U2-04 text and any companion supply or communication error codes.
- If other U2 or U4 codes are listed at the same timestamp, that points to a broader power supply interruption or wiring problem between the supply panel and the unit.
- If U2-04 appears alone, that narrows the stop to the confirmed SP-PAM undervoltage condition for the installer to measure.
- Evidence to keep:
- Keep a photograph of the [11] Malfunctioning list and the U2-04 long description screen.
Do not attempt to open the outdoor unit or indoor unit switch box cover while reviewing error codes.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Check for brownouts or heavy electrical loads running at the time of U2-04
- Who:
- You, by observing household electrical behaviour without tools.
- How:
- Note whether lights dimmed, high-power appliances were starting, or the utility company reported low voltage when U2-04 occurred.
- Result that matters:
- Establishing whether a property-wide voltage sag coincided with the SP-PAM undervoltage confirmation.
- If lights dimmed or other equipment faltered across the building, that suggests an external utility or main panel voltage drop triggered U2-04.
- If household power remained steady, that rules out a noticeable whole-house brownout and points to the dedicated heat pump feed or outdoor unit electronics.
- Evidence to keep:
- Write down the timestamp of U2-04 and any simultaneous household electrical symptoms.
Never connect the heat pump through an external timer or utility-switched circuit that regularly turns power on and off.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Verify circuit breaker levers visually and check dealer details in [6.2]
- Who:
- You, looking at the electrical cabinet and the indoor unit screen.
- How:
- Confirm without touching the levers that the heat pump circuit breakers remain ON, and open [6.2] Information > Dealer information to get the installer phone number.
- Result that matters:
- Ensuring protective routines stay powered while arranging professional voltage measurement prior to manual reset.
- If a circuit breaker has tripped OFF, that indicates a line fault or overload occurred and confirms you must not reset it before the installer finds the cause.
- If all breakers are ON, that confirms the unit still receives standby power while waiting for the technician to diagnose and manually reset U2-04.
- Evidence to keep:
- Save a photo of the breaker positions and record the date and time you contacted the installer.
Do not switch off the circuit breakers to try to clear U2-04; keeping them ON maintains the R290 leak detection and pump anti-blockage protections.
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 U2-04
- code erreur Daikin Altherma U2-04
- codice U2-04 pompa di calore Daikin
- código U2-04 bomba de calor Daikin
- Daikin Altherma error U2-04
- Daikin Altherma Fehler U2-04
- Daikin heat pump code U2-04
- Daikin U2-04
- Daikin Wärmepumpe Fehlercode U2-04
- défaut U2-04 pompe à chaleur Daikin
- error Daikin Altherma U2-04
- errore Daikin Altherma U2-04
What it means
Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 214) assigns code U2-04 to Supply voltage error with the trigger SP-PAM undervoltage confirmed. When the outdoor unit power electronics confirm an undervoltage state in the SP-PAM circuit, the controller stops heat pump operation and holds the fault until a manual reset is executed. Because this undervoltage can result from a low incoming mains voltage, excessive voltage drop across field wiring or loose switch box terminals, or an internal inverter/SP-PAM circuit fault, U2-04 by itself never proves that the circuit board is broken.
Warnings and stop conditions
- Highsource verified
Do not open the outdoor unit or measure live terminals yourself to troubleshoot U2-04: the unit operates at dangerous mains and inverter voltages and contains R290 flammable refrigerant.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Mediumsource verified
Never extend the power supply or interconnection cable with wire connectors, clamps, taped wires, or extension cords, as these cause voltage drop, overheating, electric shock, or fire.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Incoming mains voltage dropping below the unit identification label ratingCommon
A sustained brownout or low utility voltage at the property causes the outdoor unit power stage to confirm SP-PAM undervoltage when the compressor draws power.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Voltage drop across undersized cables, spliced wiring, or loose X1M terminalsCommon
Loose connections in the switch box, insufficient conductor cross-section for the rated current, or prohibited wire connectors and extension clamps cause voltage to sag under load and trigger U2-04.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Internal fault in the outdoor unit SP-PAM or inverter power electronicsPossible
If supply voltage at the outdoor unit terminals remains within specification under load yet U2-04 recurs, the SP-PAM circuit or voltage sensing on the outdoor unit PCB is malfunctioning.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does U2-04 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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