Nibe 42 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 42 fault code mean?
Fault code 42 indicates that the third phase of the electrical supply reaching the compressor has either disappeared entirely or dropped below 160 volts for a continuous period of more than 30 minutes. When this condition is detected, the heat pump locks out the compressor to prevent damage, meaning the system stops producing heat until the underlying electrical issue is identified and resolved. On the controller display a red indicator light appears alongside an on-screen alarm message showing the code number.
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
- Tripped MCB or isolator switch Common
The most frequent first finding is a tripped miniature circuit breaker in the consumer unit or a tripped outdoor-unit isolator. A transient overload or a momentary voltage dip on the supply can trip protective devices, cutting phase 3 entirely.
- Loose or corroded terminal connection Common
Over time, vibration from the compressor and thermal cycling can loosen the terminal carrying phase 3 inside the heat pump's electrical enclosure. A high-resistance or intermittent connection causes the voltage on that phase to drop below the 160 V threshold under load.
- Faulty soft-start or contactor Sometimes
The soft-start module or contactor that switches phase 3 to the compressor at startup can develop an internal fault or a burned contact, preventing the full supply voltage from reaching the compressor winding.
- Grid supply phase loss Sometimes
In rural or semi-rural areas a fault on the local distribution network can cause one of the three incoming phases to be lost or degraded at the meter. This is outside the heat pump itself but will trigger the same alarm.
- Compressor winding fault Rare
Prolonged operation with an unbalanced or weak phase can damage the compressor's internal windings, causing abnormal current draw that further depresses the voltage on that phase. A winding fault is usually discovered during engineer diagnostics after simpler causes have been ruled out.
How to fix it
- Check for tripped circuit breakers and the outdoor isolator DIY safe
Go to your consumer unit and look for any MCB or RCD that has tripped to the off position. If you find one, reset it. Next, locate the dedicated isolator for the outdoor unit (usually a red rotary switch on or near the external wall) and confirm it is fully in the ON position. Do not attempt to reset a breaker that trips again immediately — this signals a more serious fault.
- Reset the alarm on the NIBE controller DIY safe
On the display, select the alarm message to open its detail screen, then choose the option to reset the alarm and allow the unit to restart. The system will attempt to run the compressor again. If fault code 42 reappears within a short time, the underlying electrical problem has not been resolved and further DIY action will not help.
- Check around the outdoor unit for obvious physical obstructions DIY safe
While you are outside, make sure there is no debris, ice, or vegetation blocking the airflow around the outdoor unit. Although this is unlikely to be the direct cause of a phase fault, a heavily obstructed fan can contribute to electrical stress. Do not attempt to open any panel on the outdoor unit.
- Have a qualified engineer inspect the internal wiring and terminals heat pump engineer
An MCS-certified heat pump engineer should open the electrical enclosure and use a multimeter to verify the voltage on all three phases at the terminals, check torque on the phase 3 terminal block, and look for signs of arcing, burn marks or corrosion. This work involves exposed three-phase conductors and must not be attempted by a homeowner.
- Have the soft-start module and contactor tested and replaced if necessary heat pump engineer
If wiring integrity is confirmed, the engineer will test the soft-start module and the compressor contactor under load conditions. A failed soft-start or contactor is a relatively straightforward replacement for a trained engineer and is a common resolution for this fault code.
- Have the compressor windings tested heat pump engineer
If the electrical supply and switching components are all healthy, the engineer will carry out insulation-resistance and winding-continuity tests on the compressor itself. A failed compressor winding requires compressor replacement — significant work that must be completed by an MCS-certified engineer; any associated refrigerant handling requires an F-Gas registered engineer.
- Contact an MCS-certified NIBE-approved heat pump engineer heat pump engineer
If the alarm cannot be cleared after checking the MCBs and resetting, book a visit from an MCS-certified engineer who is NIBE-approved. NIBE maintains a network of approved repair partners across the UK who are factory-trained on NIBE systems. Describe the fault code and how long the unit has been offline so they can bring the most likely replacement parts.
Parts you may need
- Compressor soft-start module (NIBE compatible) · from £180
- Compressor contactor / relay · from £45
- Three-phase terminal block / busbar · from £25
- Phase-monitoring relay · from £60
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 £180–£550, depending on the underlying cause.
Frequently asked questions
Can I reset Nibe fault code 42 myself?
You can carry out the initial reset from the controller display, and it is worth checking your consumer unit for a tripped MCB first. However, if the fault returns after resetting, the cause is an electrical fault inside the heat pump or on the supply that requires a qualified engineer to diagnose safely. Three-phase electrical work is not suitable for DIY.
How long can I leave the heat pump off while waiting for an engineer?
The heat pump will not produce any heat while the compressor is locked out. In mild weather a well-insulated home may stay comfortable for a day or two using any residual heat in the buffer or underfloor circuits, but in cold weather you should treat this as an urgent call-out. Some NIBE systems can run an immersion heater element for domestic hot water as a backup — check your controller's manual operation options while you wait.
Could the problem be with my electricity supply rather than the heat pump?
Yes, it is possible. If a phase has been lost at the distribution network level, your heat pump will detect the undervoltage and raise fault code 42, but the fault lies with your DNO (Distribution Network Operator) rather than the heat pump itself. An engineer can confirm this quickly with a voltage measurement at the supply terminals. If a grid phase is missing, contact your DNO.
What does fault code 42 cost to fix, and could it mean I need a new compressor?
Most cases of fault code 42 are resolved by an engineer tightening or replacing a faulty terminal connection, or swapping a failed soft-start module or contactor. For those repairs, expect to pay roughly £180–£550 including labour and parts. A compressor winding failure is a less common but more serious outcome — if the compressor itself has been damaged by prolonged phase loss, replacement typically costs £1,500–£3,000 or more depending on the unit size and refrigerant type. This is worth mentioning to your engineer upfront so they can give you an honest assessment before committing to expensive parts.