Nibe 43 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 43 fault code mean?
Fault code 43 on a Nibe heat pump indicates that the three-phase electrical supply is connected in the wrong order. The unit's internal monitoring detects that the phase rotation is incorrect and immediately blocks the compressor from running as a protective measure. While the fault is active, the heat pump cannot produce heating or domestic hot water.
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
- Three-phase supply wired in incorrect rotation Common
The most frequent cause. During initial installation, after electrical work on the supply, or following a disturbance to the incoming wiring, two or more of the three live phase conductors are connected in the wrong order at the terminal. The heat pump's phase-sequence relay detects the reversed rotation and locks the compressor out.
- Seized or mechanically blocked compressor Sometimes
If the compressor is struggling to turn or has seized internally, it can trigger the same phase-sequence alarm even when the wiring order is correct. This is a more serious mechanical fault and will persist even after the electrical supply is checked and confirmed correct.
- Loose or disturbed supply wiring Sometimes
Vibration, a recent power outage, or work carried out in the property's consumer unit can occasionally cause a phase conductor to come loose or partially disconnect. This can alter the effective phase sequence presented to the heat pump.
How to fix it
- Check the display and note the fault code DIY safe
Confirm that code 43 is shown on the controller display alongside the alarm indicator. Take a photo for your records and note any other codes displayed at the same time, as these may help an engineer diagnose the issue more quickly.
- Attempt an alarm reset from the controller menu DIY safe
On the Nibe display module, select the alarm message and choose the option to view more information about the alarm, then select 'Reset alarm and try again'. The display will restart and the unit will attempt to resume normal operation. If code 43 returns within a short time, the underlying fault is still present and the reset alone is not sufficient.
- Check that the isolator and MCB for the heat pump have not tripped DIY safe
Go to your property's consumer unit and confirm the circuit breaker or MCB assigned to the heat pump is in the on position. Also check the local isolator switch near the outdoor unit. If anything has tripped, reset it once. If it trips again, do not reset it a second time — contact an engineer.
- Arrange for a qualified electrician to check and correct the phase sequence heat pump engineer
Correcting a phase sequence requires swapping two of the three live conductors on the incoming three-phase supply terminal. This work must only be carried out by a qualified electrician. It is not safe for a homeowner to attempt, as it involves a live three-phase supply. Once corrected, the alarm should be reset and the unit tested.
- If the fault persists after phase correction, contact an MCS-certified heat pump engineer to inspect the compressor heat pump engineer
If a qualified electrician has confirmed the phase wiring is correct and the alarm still returns, the compressor itself may be seized or failing. Compressor diagnostics and any work on the refrigerant circuit must be carried out by an MCS-certified engineer who also holds an F-Gas qualification. Contact Nibe's dedicated homeowner support line on 0330 311 2201 or request a Nibe-approved repair partner.
Parts you may need
- Three-phase phase-sequence relay / monitoring board · from £85
- Compressor (model-dependent) · from £900
- Supply wiring terminals and connectors · from £20
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Nibe fault code 43 myself?
The alarm reset from the controller menu is safe for a homeowner to try. However, the underlying fault — a reversed three-phase supply — requires a qualified electrician to correct. You should not open the electrical supply enclosure or move any wiring yourself. If the compressor is involved, an F-Gas registered, MCS-certified heat pump engineer is also needed.
How much does it cost to fix Nibe fault code 43 in the UK?
If the fault is purely a phase sequence wiring error, which is the most common cause, a qualified electrician can usually resolve it in a single visit for around £150–£350 including call-out and labour. If the compressor has seized and needs replacing, costs are significantly higher — a compressor replacement on a Nibe heat pump typically runs to around £2,000 or more depending on the model. The headline repair range above reflects the typical wiring-correction scenario that most people with this fault will face.
Why has my Nibe heat pump suddenly stopped heating and is showing fault 43?
Code 43 causes the compressor to lock out immediately, which is why the heat pump stops producing heat or hot water the moment it appears. The unit is protecting the compressor from running on a reversed phase sequence, which could cause mechanical damage. The system will remain locked out until the phase order is corrected and the alarm is manually reset.
What is the difference between Nibe fault codes 40–42 and fault code 43?
Fault codes 40, 41, and 42 are triggered when one or more compressor phases are missing entirely or have dropped below around 160 V for a sustained period — so they relate to a phase being absent or weak. Fault code 43 is different: it fires when all three phases are present but connected in the wrong rotational order. The fix for 43 is to swap two phase conductors, not to restore a missing phase.