Ravenheat CSI12 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat CSI12 fault code mean?
Fault code 12 on the Ravenheat CSI 120 range indicates a failed central-heating temperature sensor: the NTC thermistor the board relies on to know how hot the primary water is has stopped returning a valid reading, and Ravenheat's guidance is simply that the sensor has an issue and may need replacing. Without a believable temperature the board cannot modulate the burner or protect against overheating, so it suspends normal operation and shows the code. The sensor itself is a small, inexpensive component; the diagnosis is confirming whether the fault is the sensor element, its wiring, or (rarely) the board input reading it.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed heating NTC sensor Common
Years of thermal cycling eventually kill the thermistor element — it drifts badly, goes open or shorts. The board's plausibility checks reject the reading and raise code 12. A replacement sensor is the standard fix.
- Sensor wiring or connector fault Sometimes
A corroded spade connector or chafed lead mimics a failed sensor. Engineers measure at both the sensor and the board connector to tell the two apart before fitting parts.
- Scale insulating the sensor pocket Sometimes
In hard-water areas, scale on the sensor pocket slows and skews readings until they fail the board's checks. Cleaning or replacing the sensor restores accurate tracking.
- PCB input fault Rare
Rarely the measuring input on the control board fails; suspected only after a known-good sensor still reads implausibly at the board.
How to fix it
- Reset once DIY safe
A single reset shows whether the reading failure is constant. If code 12 returns as the boiler warms, the sensor circuit is genuinely faulty.
- Note any symptoms before the fault DIY safe
Radiators overshooting, the boiler cycling oddly or kettling beforehand all suggest the sensor had been drifting — useful confirmation for the engineer.
- Leave testing and replacement to an engineer Gas Safe engineer
The sensor sits on the primary pipework inside the sealed case. A Gas Safe registered engineer will measure its resistance against the temperature table, check the loom, and replace whichever element has failed.
- Engineer: verify after replacement Gas Safe engineer
After fitting, the engineer should watch the displayed/measured temperature track sensibly from cold through a full heat cycle before signing off.
Parts you may need
- Heating NTC thermistor · from £20
- Sensor wiring loom · from £25
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 £90–£250, depending on the underlying cause.
Frequently asked questions
What does code 12 mean on my Ravenheat CSI?
The central-heating temperature sensor has failed its checks — the board no longer trusts its reading of primary water temperature, so it stops normal operation. Ravenheat's guidance is that the sensor likely needs replacing, which is a quick and inexpensive job for an engineer.
Is a sensor fault urgent?
It is not dangerous — the boiler is refusing to run without trustworthy readings, which is exactly what it should do. But you will have no heating until it is fixed, and the repair is one of the cheaper ones, so book it promptly.
Can I replace the NTC sensor myself?
No — it means opening the sealed casing of a gas appliance and, on drain-down designs, working on the primary circuit. Under UK regulations that work belongs to a Gas Safe registered engineer, and it is inexpensive to have done.