Chaffoteaux 112 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 112 fault code mean?
Fault code 112 on a Chaffoteaux boiler indicates that the PCB has detected a problem with the NTC thermistor monitoring the water temperature at the inlet of the main heat exchanger. The boiler's control board is receiving either no signal at all (open circuit) or a garbled, out-of-range signal (short circuit) from this sensor, so it shuts down as a precaution to prevent overheating or unsafe operation. In plain terms, the boiler can no longer trust the temperature reading at the point where water enters the heat exchanger, and it will not fire until the issue is resolved.
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 or worn NTC thermistor Common
The thermistor itself is the most likely culprit. Over time, the resistive element inside the sensor degrades and can either break entirely (open circuit) or develop an internal short. When this happens, the resistance readings sent to the PCB fall well outside the expected range and the boiler locks out. A simple resistance test with a multimeter will confirm whether the sensor is faulty.
- Damaged or corroded wiring and connectors Common
The wiring loom between the inlet sensor and the PCB passes through a warm, sometimes damp environment inside the boiler casing. Connector pins can corrode, wires can chafe against the casing and short to earth, or a plug can work loose over time. Any break or unwanted contact in that circuit mimics a failed sensor and triggers fault 112.
- Moisture ingress at the sensor connector Sometimes
Condensation or a small water leak nearby can introduce moisture into the sensor's electrical connector. This creates a low-resistance path between pins, producing a short-circuit fault reading. If the connector is wet, drying it out and resolving the source of moisture may clear the fault without replacing the sensor itself.
- PCB fault Rare
If the sensor and all associated wiring test correctly but fault 112 persists, the control board itself may be misreading the sensor signal — for example due to a damaged analogue input circuit on the PCB following a voltage surge or component failure. This is less common but should be considered after the sensor and wiring have been ruled out.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on the boiler's front panel as described in your user manual. Fault 112 can occasionally be triggered by a brief, transient signal dropout rather than a true component failure. If the boiler fires up normally and stays running without the code returning, monitor it over the next day or two. If the code comes back, move on to the steps below.
- Check that the boiler's external electrical supply is stable DIY safe
Make sure the boiler's isolator switch is on and that no relevant circuit breakers in your consumer unit have tripped. A power interruption can sometimes cause a spurious sensor fault on restart. This takes only a moment to verify before calling an engineer.
- Have a Gas Safe engineer inspect the sensor wiring and connectors Gas Safe engineer
An engineer will isolate the boiler electrically, then trace the wiring between the heat exchanger inlet sensor and the PCB. They will check for visible damage, corrosion, chafed insulation, and loose or damp connector plugs. Many fault 112 cases are resolved at this stage simply by cleaning a connector or replacing a short section of damaged wire.
- Have the engineer test the NTC thermistor with a multimeter Gas Safe engineer
The NTC thermistor has a known resistance-to-temperature relationship. At room temperature a healthy sensor typically reads in the range of several kilohms; the resistance drops predictably as temperature rises. If the reading is open-circuit (infinite resistance) or zero ohms, the sensor has failed and needs replacing. Compatible replacement sensors for Chaffoteaux boilers are widely available and relatively inexpensive.
- Have the engineer replace the inlet NTC thermistor if faulty Gas Safe engineer
Once confirmed faulty, the engineer will fit a replacement sensor, reconnect all wiring securely, and run the boiler through a full heating cycle to verify the fault code does not return. They will also check that the new sensor's readings are tracking correctly on the boiler's diagnostic menu if one is available.
- If the sensor and wiring are both healthy, have the PCB assessed Gas Safe engineer
A correctly wired, in-specification sensor that still produces fault 112 points toward a PCB input fault. The engineer may try a known-good sensor as a swap test before concluding the PCB is the problem. PCB replacement or specialist repair is a more significant job and cost, so it is worth ruling out every other cause first.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All the diagnostic and repair work described above must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not attempt to access internal boiler components, test live wiring, or replace parts yourself.
Parts you may need
- Heat exchanger inlet NTC thermistor (sensor) · from £18
- Sensor connector plug and wiring harness section · from £22
- Chaffoteaux PCB (control board) · from £280
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
Is fault code 112 dangerous — should I keep using the boiler?
The boiler will typically lock itself out when fault 112 appears, so it will not fire while the code is active. This is a safety feature rather than a sign of immediate danger such as a gas leak. That said, you should not attempt to repeatedly reset and force the boiler to run if it keeps locking out on the same fault. Leave it off and arrange for a Gas Safe engineer to visit.
How much does it typically cost to fix Chaffoteaux fault code 112 in the UK?
For most households the repair comes in between £100 and £280. The sensor itself is inexpensive — usually £10–£25 for the part — but you are also paying for the engineer's call-out and labour, which typically runs £60–£120 depending on your region and the engineer. London and the South East tend to be 15–25% higher than the national average. If the root cause turns out to be the PCB rather than the sensor, costs rise significantly — a replacement PCB for a Chaffoteaux boiler can cost £200–£350 for the part alone plus labour, so it is worth getting the sensor and wiring checked thoroughly first.
Can I replace the NTC thermistor myself to save money?
It is strongly recommended that you do not. While the sensor itself is not a gas component, accessing and replacing it involves working inside the boiler casing with live electrical circuits and pressurised water connections. Under UK law, Gas Safe registration is not required for sensor replacement specifically, but errors during the work could damage the boiler, cause a water leak, or create an electrical hazard. More practically, most boiler manufacturers and home insurers expect service work to be carried out by a competent heating engineer. The labour cost for a straightforward sensor swap is usually modest, so the saving from a DIY attempt is small relative to the risk.
The fault cleared after a reset but keeps coming back — what does that mean?
An intermittent fault 112 that clears temporarily after a reset but returns within hours or days suggests the sensor or its wiring is on the way out rather than having failed completely. A thermistor that is beginning to break down internally can produce correct readings when cold but drop out once it warms up, or it may have a hairline break in its wiring that only opens under vibration or thermal expansion. An engineer can monitor the sensor's live resistance readings through a heating cycle to catch intermittent faults that would not show up on a cold static test.