Nibe 1 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 1 fault code mean?
Nibe Alarm Code 1 indicates that the heat pump's outdoor temperature sensor, known as BT1, is either not connected or has been detected as defective. The controller expects a continuous, valid reading from this sensor to calculate the correct flow temperature for your heating system. When the BT1 reading is absent or falls outside the expected range — typically due to an open circuit or short circuit on the sensor input — the unit raises Alarm 1 and falls back to a minimum calculated flow temperature. A red indicator light will appear on the top right of the indoor unit display alongside the alarm message.
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
- Loose or disconnected BT1 sensor plug Common
The BT1 outdoor temperature sensor connects to the controller via a plug and cable. Vibration, accidental knocks during maintenance visits, or simply age can allow the plug to work loose, breaking the electrical contact and triggering the open-circuit alarm. This is the first thing an engineer will check.
- Damaged or broken sensor cable Common
The cable running from the outdoor sensor to the controller can be pinched, chafed against pipework, or damaged by rodents or during building work. Even a partial break in the cable can cause intermittent or permanent loss of the sensor signal.
- Failed BT1 outdoor temperature sensor Sometimes
The sensor itself contains a thermistor that can degrade or fail outright over time, particularly if it has been exposed to prolonged damp or physical damage. A failed thermistor will present as an open or short circuit to the controller.
- Electrical surge or lightning strike damaging the control board Sometimes
A voltage spike — for example from a nearby lightning strike — can damage the sensor input circuitry on the main PCB, causing the controller to report a sensor fault even when the sensor and cabling are undamaged.
- Corrosion or damage on the control board sensor terminals Rare
Moisture ingress into the indoor unit or prolonged damp conditions can cause corrosion to develop on the PCB terminals or connectors associated with the BT1 input, resulting in a high-resistance or open-circuit reading.
How to fix it
- Reset the alarm from the controller display DIY safe
On the Nibe display, select the Alarm 1 message and choose the option to reset the alarm and try again. The display module will restart and the heat pump will attempt to resume normal operation. If the alarm clears and does not return, monitor the system over the next 24 hours to confirm it stays away.
- Power-cycle the entire system DIY safe
If the soft reset does not clear the alarm, switch off the heat pump at the main isolator (usually located near the indoor unit) and also trip the MCB for the heat pump on your consumer unit. Leave everything off for around five minutes to allow the controller capacitors to fully discharge, then restore power. This can clear transient communication errors that mimic sensor faults.
- Visually inspect the BT1 sensor and its cable run DIY safe
Locate the BT1 outdoor temperature sensor — it is typically mounted on a north- or north-west-facing wall in a shaded position. Without touching any electrical terminals, visually follow the cable back towards the indoor unit looking for obvious signs of damage: crushed sections, chafed insulation, kinks, or areas where the cable may have been disturbed by recent building work. Also check that the plug at the controller end is firmly seated. Do not open the controller enclosure or touch any internal wiring.
- Have an engineer test the BT1 sensor resistance and wiring continuity heat pump engineer
A Nibe-approved or MCS-certified heat pump engineer will use a multimeter to measure the resistance across the BT1 thermistor at the controller terminal block and compare it against the expected values in the service documentation. They will also test for continuity and insulation resistance along the cable run to pinpoint whether the fault lies in the sensor head, the cable, or the PCB input.
- Replace the BT1 outdoor temperature sensor if confirmed faulty heat pump engineer
If testing confirms the sensor has failed, the engineer will fit a replacement BT1 sensor. The part itself is low cost, so this is typically one of the more straightforward and affordable Nibe repairs. The engineer will re-run and secure the cable correctly and confirm the controller is reading a plausible outdoor temperature before leaving.
- Inspect and repair or replace the PCB if the sensor and cabling test healthy heat pump engineer
If the BT1 sensor and its entire cable run test within specification but the alarm persists, the fault lies on the control board itself — most likely corrosion or surge damage to the sensor input. Board repair or replacement must be carried out by a qualified engineer. Contact Nibe's homeowner care line on 0330 311 2201 to arrange a Nibe-approved repair partner visit, which will help protect any remaining warranty.
Parts you may need
- Nibe BT1 outdoor temperature sensor · from £35
- Sensor cable and connector set · from £20
- Nibe main control PCB (model-dependent) · from £290
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–£380, depending on the underlying cause.
Frequently asked questions
Will my heat pump still provide heating while Alarm 1 is showing?
In many cases, yes — Nibe is designed to fall back to a minimum calculated flow temperature when it loses the BT1 outdoor sensor reading, so the system may continue to run at a reduced or fixed output rather than shutting down completely. However, without the outdoor sensor, weather compensation cannot work properly, meaning the heat pump may be less efficient and your home may not reach the desired temperature in very cold weather. It is worth getting the fault resolved promptly rather than leaving the system running in fallback mode indefinitely.
Can I fix Nibe Alarm 1 myself?
The checks a homeowner can safely carry out are limited to: resetting the alarm via the display, power-cycling the unit at the isolator, and visually inspecting the sensor cable for obvious physical damage. If the alarm clears after a reset and does not return, no further action may be needed. If the alarm persists, do not open the controller enclosure or attempt to test or replace the sensor yourself — call an MCS-certified heat pump engineer or a Nibe-approved repair partner.
How much does it cost to fix a Nibe BT1 sensor fault in the UK?
For the majority of cases — a loose connection, damaged cable, or failed sensor — most UK homeowners pay in the region of £150 to £380 including parts and labour. The BT1 sensor itself is an inexpensive component (typically £25–£50), so labour makes up most of the bill. If the fault has been caused by a surge that has damaged the main PCB, a board replacement will add significantly more to the cost; PCBs for Nibe units typically run £200–£350 for the part alone, so a full repair including labour could exceed £500 in that scenario.
Where exactly is the BT1 outdoor sensor fitted on a Nibe heat pump?
The BT1 sensor is a small wired temperature probe mounted externally on the building — typically on a north- or north-west-facing wall away from direct sunlight and sheltered from driving rain where possible. It is not part of the outdoor heat pump unit itself; it is a separate wall-mounted probe connected back to the indoor controller by a thin cable. Because it is outside and exposed to the elements, its cable can be vulnerable to weathering, rodent damage, or disturbance during external building work.