Nibe 16 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 16 fault code mean?
Alarm 16 on a NIBE heat pump reports a fault on temperature sensor BT16 — the evaporator sensor. NIBE's alarm list gives the cause plainly: sensor not connected or defective. BT16 watches the evaporator, the heat exchanger where the refrigerant absorbs heat, and its reading drives defrost decisions and evaporator protection; NIBE also uses it for the related alarm 159 (evaporator temperature too high). Alarm 16 is listed with automatic reset — it clears by itself if the reading returns — so a one-off may be a connector or a transient. A persisting or recurring alarm 16 means the sensor or its wiring has genuinely failed, and the standard thermistor routine applies: resistance check, then replace sensor or repair loom.
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
- BT16 sensor defective Common
The evaporator thermistor fails open, short or drifted — NIBE's documented cause, confirmed against the resistance table.
- Sensor not connected / connector fault Common
The alarm text names 'not connected' explicitly: a dislodged or corroded connector produces the identical symptom and is checked first.
- Wiring damage Sometimes
A chafed or pinched lead between sensor and board mimics sensor failure; proven before parts are fitted.
How to fix it
- Acknowledge the alarm once and observe DIY safe
On the display, select the alarm and choose to reset/acknowledge it (or use 'aux. operation' if offered, which keeps some heating running). One reset is legitimate; if the alarm returns, leave it for an engineer rather than resetting repeatedly.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will check BT16's connector and wiring, measure its resistance against NIBE's sensor table, and replace the sensor or repair the loom as found — then confirm defrost behaviour is sensible with a valid evaporator reading restored. Any work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; a NIBE-accredited (NIBE Pro) installer is the natural choice.
Parts you may need
- BT16 evaporator temperature sensor · from £30
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 £100–£280, depending on the underlying cause.
Frequently asked questions
What does alarm 16 mean on my NIBE?
Temperature sensor BT16 — the evaporator sensor — is not connected or has failed. It guides defrosting, so the unit cannot manage frost properly without it. The alarm auto-resets if the reading returns; a persisting alarm means the sensor or wiring needs testing and replacing.
The alarm comes and goes — is that still a fault?
Yes — an intermittent alarm 16 is the signature of a failing connector or a sensor drifting in and out of range. Auto-reset clears the display, not the cause; have it looked at before winter, when defrost control matters most.
Is this related to alarm 159?
Closely — 159 fires when BT16's reading exceeds 50°C (evaporator too hot), while 16 fires when BT16 itself fails. A faulty sensor can produce either; the engineer's resistance check distinguishes a lying sensor from a real temperature problem.