Daikin U2-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin U2-00 fault code mean?
The U2-00 fault code on Daikin Altherma heat pumps indicates that the unit has detected an abnormal condition in its electrical power supply. This could be an open phase, a voltage imbalance, an instantaneous power failure, or a voltage level that has dropped or risen beyond the safe operating range. The control board continuously monitors incoming supply voltage and DC bus voltage, and when either strays outside acceptable limits, it locks the unit out and displays U2-00 to prevent damage to internal components such as the compressor and PCB. In straightforward cases this clears after a brief reset; in persistent cases it points to a wiring, fusing, protection board, or PCB fault requiring a qualified engineer.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Momentary grid voltage fluctuation or power cut Common
A brief brown-out, voltage dip, or power interruption on the mains supply is the most frequent trigger for U2-00. The protection circuit trips before any hardware is damaged, and the fault often clears with a simple reset once stable mains power is restored.
- Open phase or persistent voltage imbalance Common
On three-phase installations the supply must be balanced across all phases. If one phase is lost or the voltage differential between phases exceeds roughly 10% of the rated value, the unit treats this as an open-phase event and locks out. Single-phase units can also show this if supply voltage drifts significantly outside the rated band.
- Loose or deteriorated wiring connections Sometimes
Loose terminals at the outdoor unit's main terminal block, or corrosion and heat discolouration on cable connections, can cause intermittent voltage drops that mimic a supply fault. Vibration over time can work terminals loose even on well-installed systems.
- Blown fuse in the outdoor unit isolator or disconnect box Sometimes
The outdoor unit is normally protected by fuses in a local fused isolator. If one or more fuses have blown — due to a surge, age, or an underlying fault — the unit will see an open phase or loss of supply and display U2-00.
- Faulty phase-protection relay or board Sometimes
The phase-protection board is a small dedicated PCB that monitors supply phases and opens safety circuits if it detects a phase loss. These boards can fail themselves, causing a spurious U2-00 even when the supply is perfectly healthy. Once other causes are ruled out, this is the next most likely component.
- Failed or damaged main PCB Rare
If the power supply and all wiring check out correctly, the fault can originate inside the control PCB itself — either at the DC bus voltage detection circuitry or at a component that has failed due to a surge or age. PCB failure is less common but results in a persistent U2-00 that does not clear with resets.
How to fix it
- Check for a local power cut or grid event DIY safe
Before doing anything else, check whether neighbouring properties have power and whether any other appliances in your home are affected. If there has been a wider power interruption, the U2-00 may clear on its own once stable supply is restored.
- Inspect the consumer unit for a tripped MCB or RCD DIY safe
Go to your consumer unit (fuse board) and check whether the circuit breaker or RCD feeding the heat pump has tripped. If it has, reset it once. If it trips again immediately, do not attempt further resets — call an engineer, as a recurring trip indicates a fault on the circuit.
- Check and reset the outdoor unit isolator switch DIY safe
Locate the local isolation switch or fused spur mounted near the outdoor unit. Switch it off, wait 30 seconds, and switch it back on. Confirm the isolator itself has not tripped. Do not open the isolator enclosure or touch any internal wiring.
- Power cycle the heat pump from the controller DIY safe
Turn the system off at the indoor controller or room thermostat, then switch off the main power to the unit and leave it off for 10–15 minutes. This allows the DC bus capacitors to discharge fully and the protection circuit to reset. Then restore power and restart. If U2-00 does not return, monitor the unit over the next 24–48 hours for recurrence.
- Clear any obstructions around the outdoor unit DIY safe
While you are outside, check that the outdoor unit is free from snow, ice, leaves, or debris that could restrict airflow. A heavily restricted unit can draw higher current and stress the power supply, though this is a secondary concern for U2-00 specifically.
- Check system water pressure if the display allows DIY safe
On the Altherma controller, confirm the system water pressure is within the normal range (typically 1.0–1.5 bar for most domestic systems). Low pressure can cause other faults to appear alongside U2-00. Top up via the filling loop if pressure is below 0.8 bar, following your system's manual.
- If the fault persists, contact an MCS-certified heat pump engineer heat pump engineer
A persistent or recurring U2-00 requires professional diagnosis. The engineer will measure incoming supply voltage across all phases with a meter, inspect and tighten terminal connections, test the fuses in the outdoor unit isolator, and assess the phase-protection board and main PCB. Do not continue resetting the unit repeatedly, as this can mask a developing fault and increase the eventual repair cost. Any work inside the outdoor unit, on the PCB, or on the refrigerant circuit must be carried out by a suitably qualified engineer — refrigerant circuit work additionally requires an F-Gas registered engineer.
Parts you may need
- Phase protection relay / monitoring board · from £85
- Outdoor unit main PCB (Altherma series) · from £420
- Fused outdoor unit isolator / disconnect fuses · from £35
- Main power terminal block and wiring harness · from £55
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£450, depending on the underlying cause.
Frequently asked questions
Can I fix the Daikin U2-00 fault myself?
In some cases, yes — if the cause is simply a brief power cut or a tripped circuit breaker, a homeowner can safely reset the MCB, power cycle the unit, and clear the fault. However, any diagnosis or repair beyond these basic checks — including opening the outdoor unit, testing electrical connections, replacing fuses inside the unit, or touching the control board — must be carried out by a qualified engineer. Repeatedly resetting a persistent U2-00 without finding the root cause risks damaging the compressor or PCB, which are far more expensive to replace.
How much does it typically cost to fix a Daikin U2-00 in the UK?
If the fault clears with a reset after a power cut, there is no repair cost at all. For an engineer visit to diagnose and fix a wiring or fusing issue, most UK homeowners pay in the region of £120–£300 including callout and labour. Replacing a faulty phase-protection board typically adds £80–£120 for the part. A main PCB replacement is significantly more — parts alone can run £350–£550 for Altherma units, pushing the total bill to £600–£900 or more with labour. Compressor replacement is rarely caused by U2-00 alone but can exceed £1,500 if the unit has been left running with a known electrical fault.
Why does my Daikin Altherma keep showing U2-00 after I reset it?
A fault that returns within hours or days of resetting almost always points to an underlying hardware issue rather than a one-off grid event. The most common culprits for recurring U2-00 are a loose mains connection at the outdoor unit terminal block, a degraded fuse, or a failing phase-protection board. Less commonly, the main PCB itself has developed a fault in its voltage-monitoring circuitry. An MCS-certified engineer with a suitable multimeter can usually pinpoint the cause within an hour of inspection.
Does the Daikin warranty cover a U2-00 fault?
Daikin's standard residential warranty covers manufacturing defects on parts for three years (some registered installations extend this). However, Daikin requires evidence of annual servicing by a suitably qualified engineer to honour warranty claims — without a service record, the claim is likely to be declined. If your unit is within warranty and has been serviced annually, contact your installing engineer or Daikin UK directly before authorising any paid repair, as covered faults should be rectified at no parts cost to you.