Protherm F.10 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.10 fault code mean?
The F.10 fault code on most Protherm boilers — particularly the Gepard (Cheetah) range — points to a short circuit detected in the central heating flow NTC temperature sensor (sometimes labelled NTC2). The boiler's control system monitors the voltage signal from this sensor, and when it drops below roughly 0.45 V, the boiler shuts down as a safety precaution. In plain terms, the sensor is either sending a bad signal or has failed outright, so the boiler can no longer safely track water temperature in the heating circuit. Note that on the Protherm Lynx Combi, F.10 carries a different meaning — it signals a system water pressure error rather than a sensor fault, so always check which model you have before beginning any diagnosis.
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 or failed CH flow NTC sensor Common
The NTC2 flow temperature sensor itself is the most common culprit. Over time, the thermistor element inside degrades or develops an internal short circuit, causing the signal voltage to fall outside the acceptable range. The boiler's control board detects this and locks out immediately.
- Damaged or corroded wiring and connectors Common
The wiring loom running between the NTC sensor and the control board can suffer from heat damage, cracked insulation, or oxidised connector pins — any of which can cause a voltage drop that mimics a shorted sensor. Oxidation on the connector at either end is a particularly common finding during engineer inspections.
- Unstable mains power supply Sometimes
Voltage spikes or sustained undervoltage from the mains can interfere with low-level sensor signal circuits, causing the control board to register a false short-circuit reading. This is a recognised cause of intermittent F.10 errors that clear on their own after a reset.
- PCB (control board) fault Rare
If the sensor and wiring both check out correctly, the control board itself may have developed a fault — typically a damaged input circuit for the NTC signal. Burnt tracks, failed components, or moisture ingress on the board can all cause the same symptom.
How to fix it
- Check whether the boiler resets itself DIY safe
Protherm boilers can sometimes clear an F.10 fault automatically after around 10 seconds if the error was caused by a brief power fluctuation. Press the reset button (refer to your boiler's handbook for its exact location) and wait to see if the boiler fires up normally. Attempt this no more than two or three times in total — repeated failed resets suggest a genuine component fault that needs an engineer.
- Verify your mains electricity supply DIY safe
Check that no fuses have tripped in your consumer unit and that other appliances in the property are working normally. An unstable or interrupted power supply can trigger spurious sensor faults on Protherm boilers. If you notice frequent power fluctuations, mention this to the engineer — fitting a voltage stabiliser or UPS can help prevent repeat occurrences.
- If you have a Protherm Lynx Combi, check your system pressure DIY safe
On the Lynx Combi, F.10 indicates a water pressure fault rather than a sensor issue. Check the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, top up via the filling loop following the instructions in your boiler's handbook, then attempt a reset. If pressure drops repeatedly, an engineer needs to investigate for a leak.
- Have an engineer test the NTC flow sensor resistance Gas Safe engineer
A Gas Safe engineer will disconnect the NTC2 sensor and measure its resistance with a multimeter, comparing the reading against the manufacturer's reference values at a known water temperature. A reading outside the expected range confirms the sensor has failed and needs replacing. This work requires draining relevant parts of the system and handling electrical components inside the boiler.
- Have the wiring loom and connectors inspected Gas Safe engineer
The engineer will trace the wiring between the NTC sensor and the PCB, looking for signs of heat damage, cracked insulation, broken conductors, or corroded connector pins. Any damaged sections will be repaired or replaced. Oxidised contacts can sometimes be cleaned and treated rather than requiring full replacement.
- Have the PCB inspected if the sensor and wiring are sound Gas Safe engineer
If a new, verified-good NTC sensor and intact wiring are in place but the F.10 fault persists, the engineer will carry out a visual inspection of the control board for burnt components, cracked solder joints, or damaged signal-input circuitry. Depending on findings, the board may be repairable by a specialist or may need full replacement.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has not cleared after a reset or two, contact a Gas Safe registered engineer to carry out a proper diagnosis. Protherm is a less common brand in the UK, so try to find an engineer with experience of the range or one who is comfortable sourcing and working with Protherm parts. You can verify any engineer's registration at GasSafeRegister.co.uk.
Parts you may need
- Protherm CH flow NTC temperature sensor (NTC2) · from £20
- Sensor wiring loom / connector harness · from £35
- Protherm PCB / control board · from £250
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 the Protherm F.10 fault myself?
The only safe DIY actions are attempting a boiler reset (up to two or three times), checking your mains power supply, and — on a Lynx Combi — topping up system pressure if it has dropped. The actual diagnosis and repair of the NTC sensor, wiring loom, or PCB must be carried out by a Gas Safe registered engineer. Working inside the boiler's electrical components without the correct qualifications is unsafe and may invalidate your warranty or home insurance.
How much does it cost to fix a Protherm F.10 fault in the UK?
Most people with this fault end up paying somewhere between £120 and £320 in total. If the NTC sensor itself is at fault, the part costs around £15–£25 and the engineer's call-out and labour makes up the rest of the bill. If the PCB needs replacing, costs can rise to £400–£550 or more depending on your location and the engineer's rates — London and the South East typically run 20–40% higher than the UK average. Protherm parts may also take longer to source than those for more common UK brands, so factor in possible waiting time.
Why does my Protherm show F.10 but then clear on its own?
Intermittent F.10 faults that disappear after a reset or a brief wait are often caused by a spike or dip in the mains electricity supply rather than a faulty sensor. The control board momentarily misreads the sensor signal as a short circuit and locks out as a precaution. If this happens only occasionally, connecting the boiler via a voltage stabiliser can help. If it is happening frequently or the fault no longer clears, the sensor or wiring should be tested by an engineer before the fault becomes a permanent lockout.
My boiler is a Protherm Lynx Combi and shows F.10 — is it the same problem?
No — on the Lynx Combi, F.10 means a system water pressure error, not a sensor short circuit. Start by checking the pressure gauge: it should read between 1.0 and 1.5 bar when cold. If pressure is low, top it up using the filling loop following your handbook. If pressure keeps dropping, there is likely a leak somewhere in the system — either internally in the boiler or in the pipework — and a Gas Safe engineer will need to trace and fix it.