Chaffoteaux 712 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 712 fault code mean?
Fault code 712 on a Chaffoteaux boiler signals that the control board has detected a problem with the return water temperature sensor on Zone 2 of a multi-zone heating system. The sensor — a thermistor — may be producing a reading that falls outside the expected range, or the boiler's electronics have detected either an open circuit (a broken or disconnected wire) or a short circuit (a direct electrical fault within the sensor or its wiring). Until the issue is resolved, the boiler cannot reliably monitor the return temperature on Zone 2, which prevents safe, accurate heating control. This code sits within a wider group of multi-zone sensor faults: codes 701–703 relate to flow sensor anomalies on Zones 1–3, while codes 711–713 cover return sensor anomalies on the same zones. Code 711 therefore points to Zone 1's return sensor, 712 to Zone 2, and 713 to Zone 3. If your boiler is a single-zone combi rather than a multi-zone system, code 712 may carry a different meaning — always cross-check with your specific model's documentation.
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
- Faulty Zone 2 return thermistor Common
The thermistor itself has degraded or failed completely, causing it to return a resistance value that sits outside the acceptable range for the boiler's control board. This is the most common trigger for code 712. Thermistors can fail gradually due to heat cycling and age, or suddenly due to moisture ingress.
- Loose or corroded wiring connection Common
The wiring between the Zone 2 return sensor and the PCB uses crimp or Lucar-type connectors that can work loose over time or corrode — especially in damp or poorly ventilated plant rooms. A poor connection produces an intermittent or absent signal, which the boiler interprets as a sensor fault.
- Open or short circuit in the sensor circuit Sometimes
A broken wire anywhere in the sensor circuit creates an open circuit, while damaged insulation allowing two conductors to touch creates a short circuit. Both conditions will trigger the 712 fault. Physical damage during previous servicing or rodent damage to cables are occasional culprits.
- PCB fault Rare
In rare cases the main printed circuit board is the source of the problem rather than the sensor itself. If a PCB input channel has failed, it will misread or fail to read the sensor signal even when the thermistor and wiring test as healthy.
How to fix it
- Reset the boiler once DIY safe
Press the boiler's reset button as described in your user manual. A single reset can clear a transient fault. If the 712 code returns shortly afterwards, do not keep resetting — repeated resets without resolving the underlying fault can cause further damage. Move on to the next step.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler. It should read between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop until it reaches the correct level, then attempt another reset. Low pressure alone will not cause a 712 fault, but it is worth ruling out as a contributing system issue before calling an engineer.
- Inspect accessible wiring around the Zone 2 return sensor (visual only) DIY safe
If you can safely access the area where the Zone 2 return sensor is located without removing boiler casing panels, visually check the visible cable runs for obvious damage such as burn marks, kinking, or chafing. Do not remove any boiler panels or touch any electrical components — this is a visual check only. Report any findings to your engineer.
- Arrange for a Gas Safe engineer to test the thermistor Gas Safe engineer
A registered engineer will use a multimeter to measure the thermistor's resistance and compare it against the manufacturer's expected values at the current water temperature. If the reading falls outside the specified range, the sensor is confirmed faulty and will need replacing.
- Replace the Zone 2 return thermistor if confirmed faulty Gas Safe engineer
Your engineer will source a compatible replacement thermistor, drain down the relevant circuit section if needed, swap the sensor, and recheck the circuit before firing the boiler. They will clear the fault code and verify that the Zone 2 return temperature is now reading correctly.
- Investigate the PCB if the fault persists after sensor replacement Gas Safe engineer
If replacing the thermistor and rectifying any wiring issues does not clear the 712 code, the engineer will investigate whether the PCB's sensor input channel is faulty. PCB replacement is a more significant repair and your engineer may advise whether it represents good value given the boiler's age.
- Call a Gas Safe registered engineer if the fault does not clear after a single reset Gas Safe engineer
All diagnostic and repair work beyond a simple reset or pressure top-up must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk before booking.
Parts you may need
- Zone 2 return thermistor (NTC temperature sensor) · from £20
- Sensor wiring harness / loom · from £35
- Main PCB · from £220
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Chaffoteaux fault code 712 myself?
The safe DIY actions are limited to a single boiler reset and checking system pressure. Everything beyond that — testing the thermistor, inspecting wiring inside the boiler casing, and replacing components — must be done by a Gas Safe registered engineer. Attempting gas boiler repairs without registration is illegal in the UK and potentially dangerous.
How much will it cost to repair a Chaffoteaux 712 fault?
For most homeowners, the repair involves a thermistor replacement and wiring inspection, which typically costs between £120 and £320 including parts and labour. If the PCB turns out to be at fault, expect the bill to rise to £350–£500 or more depending on your location and engineer. London and the South East generally attract higher call-out rates than other regions.
What is the difference between fault codes 711, 712, and 713 on a Chaffoteaux boiler?
These three codes all refer to return temperature sensor anomalies in a multi-zone system, but each targets a different zone: 711 is the Zone 1 return sensor, 712 is Zone 2, and 713 is Zone 3. The cause and repair approach are essentially the same for all three — a faulty thermistor, a wiring issue, or in rare cases a PCB fault. Your engineer's first step will be to identify which zone's sensor circuit has the problem.
My Chaffoteaux boiler is over 12 years old and showing a 712 fault — is it worth repairing?
If a straightforward sensor swap resolves the issue, repair is almost always worthwhile. However, if the PCB also needs replacing and the boiler is over 12–15 years old, the economics shift — a new A-rated boiler could actually be more cost-effective over the next five years than investing £350–£500 in an ageing appliance. Ask your engineer to give you an honest assessment before authorising major parts replacements on an older boiler.