Chaffoteaux 107 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 107 fault code mean?
Fault code 107 on Chaffoteaux boilers points to a problem with water temperature sensing on the central heating flow circuit. The exact interpretation depends on your model: on newer units running the Expert Control system, it is described as a Flow Check Failed condition, meaning the boiler cannot confirm adequate water circulation. On older models such as the Britony Combi SE, code 107 corresponds to a central heating flow thermistor fault — either an open circuit (broken connection) or a short circuit within the sensor or its wiring. In both cases the underlying issue is the same: the boiler's control board is not receiving a reliable temperature signal from the flow sensor, so it shuts down as a safety precaution. Always cross-reference with your specific model's manual to confirm which interpretation applies to your unit.
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 NTC flow thermistor Common
The thermistor is a small temperature-sensitive resistor clipped onto the flow pipe. Over time it can fail internally, producing a resistance reading that is either wildly inaccurate, stuck at zero ohms (short circuit) or infinite (open circuit). The PCB detects the implausible reading and locks the boiler out with code 107. This is the most frequent cause of this fault.
- Damaged or corroded wiring and connectors Common
The two-wire loom running from the thermistor back to the PCB passes through a warm, occasionally damp environment. Connectors can oxidise, terminals can work loose, and insulation can chafe against metalwork. Any break or high-resistance joint in the circuit mimics a failed sensor and triggers the same fault code.
- Intermittent loose connection at the PCB Sometimes
Even if the wiring along its run looks intact, the plug-in connector at the board itself can develop an intermittent contact. The fault may clear on reset but return after the boiler heats up and components expand slightly, making diagnosis tricky.
- Mains supply voltage issue Rare
Chaffoteaux documentation flags incorrect mains voltage as a trigger for error 107. Significant voltage fluctuations or an unstable supply can confuse the control board and produce spurious sensor-related fault codes, even when the thermistor and wiring are physically sound.
How to fix it
- Reset the boiler DIY safe
Locate the reset button on the boiler's front panel and hold it for the duration specified in your user manual (typically 3–5 seconds). One or two resets are reasonable; if the code returns immediately or after a short run, move on rather than repeatedly resetting, as this will not resolve an underlying fault.
- Check your gas and electricity supplies DIY safe
Confirm the gas isolation valve serving the boiler is fully open and that other gas appliances in the property are working normally. Also check the boiler's fused spur or plug has not tripped. An unstable mains supply can occasionally trigger code 107, so if you suspect voltage fluctuations, note whether other appliances are behaving oddly and mention it to the engineer.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler — it should normally read between 1.0 and 1.5 bar when cold. If it is below 0.5 bar, top up via the filling loop following the instructions in your user manual, then attempt another reset. Very low pressure can prevent adequate water circulation and contribute to flow-related faults.
- Inspect the flow thermistor and its wiring (engineer) Gas Safe engineer
A Gas Safe engineer will locate the NTC thermistor on the central heating flow pipe and test it with a multimeter set to resistance mode. At around 25 °C a healthy Chaffoteaux flow thermistor typically reads 8.1–8.6 kΩ. A reading of 0 Ω indicates a short circuit; an open-loop reading confirms a break. The engineer will also check the wiring loom and PCB connector for corrosion, chafing, or loose terminals.
- Replace the NTC flow thermistor if faulty (engineer) Gas Safe engineer
If the sensor fails the resistance test, the engineer will fit a replacement. It is important that thermal paste is applied correctly when seating the new thermistor so that it reads the pipe temperature accurately. Using the correct Chaffoteaux-compatible part matters — generic sensors with the wrong resistance curve can cause the fault to persist or lead to incorrect temperature control.
- Investigate PCB or mains voltage issues if the fault persists (engineer) Gas Safe engineer
If the thermistor and wiring both test as healthy, the fault lies either in the PCB's sensor input circuit or in the mains supply reaching the board. The engineer may test mains voltage at the boiler terminals and check for fluctuations. PCB repair or replacement is a more involved and costly intervention that should only be undertaken once simpler causes have been ruled out.
- Call a Gas Safe registered engineer if the code has not cleared Gas Safe engineer
Any work inside the boiler casing — including testing components, replacing the thermistor, and certainly any work on the PCB or gas circuit — 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 open the boiler casing yourself.
Parts you may need
- Chaffoteaux / Ariston NTC flow thermistor · from £15
- Thermistor wiring loom / harness · from £25
- PCB connector / terminal repair kit · from £10
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–£250, depending on the underlying cause.
Frequently asked questions
Is fault code 107 the same on all Chaffoteaux boilers?
Not exactly. On newer Chaffoteaux models using the Expert Control system, code 107 is labelled 'Flow Check Failed' and relates to the boiler being unable to confirm proper water circulation. On older models such as the Britony Combi SE, the same code number indicates a central heating flow thermistor fault — either an open or short circuit in the sensor wiring. The practical fix often overlaps (testing and potentially replacing the flow thermistor), but it is worth checking your specific model's manual or asking your engineer to confirm which definition applies to your boiler.
Can I fix Chaffoteaux fault code 107 myself?
Homeowner checks are limited to resetting the boiler, confirming the gas and electricity supplies are on, and topping up system pressure if it has dropped. The actual diagnosis and repair — testing the thermistor with a multimeter, inspecting the wiring, and replacing any components inside the boiler casing — must be done by a Gas Safe registered engineer. Working inside a gas boiler without registration is illegal in the UK and potentially dangerous.
How much does it cost to fix fault code 107 on a Chaffoteaux boiler?
For the most common repair — a Gas Safe engineer diagnosing and replacing the NTC flow thermistor — you should expect to pay roughly £100 to £250 including call-out, labour, parts, and VAT. The thermistor itself is an inexpensive component (typically £10–£40 for the part), so the majority of the cost is labour. Prices in London and the South East tend to run 15–25% higher than the national average. If the fault turns out to involve the PCB rather than the sensor, costs can rise significantly beyond this range — your engineer should advise you before proceeding with that work.
Why does my Chaffoteaux boiler keep showing code 107 after I reset it?
A fault that clears on reset but quickly returns suggests the underlying cause — most likely a thermistor that has partially failed or an intermittent wiring connection — is still present. Repeatedly resetting the boiler will not cure it and may mask the problem. The boiler is locking out to protect itself and the heating system. Book a Gas Safe engineer to carry out a proper diagnosis; continuing to reset without investigation is not recommended.