Protherm F.71 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.71 fault code mean?
The F.71 fault code appears on Protherm boilers when the control board detects that the flow temperature sensor (the heating flow NTC thermistor) is reporting a fixed, unchanging value. Under normal operation, this sensor measures the temperature of water leaving the heat exchanger on its way to the heating circuit, and its resistance reading should change as water heats up. When the PCB sees no change in that reading after the burner fires — suggesting the sensor is stuck at a constant value — it raises F.71 and locks out the boiler as a precaution. The NTC thermistor itself should measure approximately 12 kΩ resistance at 20°C; a reading well outside that range, or a completely static signal, confirms a sensor problem. The fault is most often traced to a faulty or displaced sensor, a wiring issue, or — less commonly — a failed PCB.
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 flow NTC thermistor Common
The flow NTC sensor itself has either failed completely or is producing a signal that never changes, regardless of water temperature. This is the single most common explanation for F.71. Sensors degrade over time, particularly in hard-water areas where limescale can coat the sensor body and interfere with accurate temperature sensing.
- Sensor incorrectly positioned or dislodged Common
The flow NTC thermistor must sit firmly against the flow pipe or in its designated pocket on the heat exchanger. If it has slipped, been knocked loose during maintenance, or was never seated correctly, it will read ambient temperature rather than flow temperature — producing exactly the constant-value signal that triggers F.71.
- Damaged or disconnected wiring Sometimes
The wiring harness running between the flow NTC sensor and the PCB may have a broken connection, a chafed cable, or a short circuit. Even a slightly loose plug at either end can cause the PCB to see an unchanging signal. Engineers will check continuity and resistance across the full circuit before condemning the sensor itself.
- Limescale build-up on the sensor Sometimes
In hard-water regions of the UK, mineral deposits can accumulate on the sensor body, effectively insulating it from the water temperature. The sensor then reads a fixed or very slow-changing value, which the PCB interprets as a constant-value fault and displays F.71.
- PCB (control board) fault Rare
If the sensor, its wiring, and all connections test within specification but the fault persists, the PCB itself may be misreading or failing to process the sensor signal correctly. This is a less common but more costly outcome, and it requires an engineer to confirm by substitution or specialist testing.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Protherm boiler (usually marked with a flame or reset symbol) and hold it for a few seconds until the boiler attempts to restart. If the boiler fires up and runs normally, monitor it closely over the next hour. If F.71 returns promptly, do not keep resetting — repeated resets without fixing the underlying fault can mask a genuine problem or cause further damage.
- Check the gas supply to your home DIY safe
Confirm that your gas supply is working by checking whether other gas appliances (hob, cooker) are functioning normally. If there is a wider gas supply issue, contact your gas network operator. This step rules out an unrelated supply interruption before an engineer visits.
- Inspect wiring connections visually (engineer) Gas Safe engineer
A Gas Safe engineer will open the boiler casing and visually inspect the wiring harness running to the flow NTC sensor. They will look for loose plugs, corroded terminals, chafed insulation, or any sign of a short circuit. This is a straightforward first diagnostic step that takes only a few minutes.
- Test sensor resistance and positioning (engineer) Gas Safe engineer
The engineer will use a multimeter to check the resistance of the flow NTC thermistor — it should read around 12 kΩ at 20°C. A reading that is wildly out of range, open circuit, or completely static confirms a faulty sensor. They will also check that the sensor is correctly seated in its mounting pocket or clipped firmly to the flow pipe.
- Replace the flow NTC thermistor if required (engineer) Gas Safe engineer
If the sensor tests as faulty or is found to be damaged, the engineer will fit a new flow NTC thermistor compatible with your Protherm model. After replacement they will clear the fault, restart the boiler, and confirm that the PCB is now receiving a changing temperature signal as the system heats up.
- Replace the wiring harness if required (engineer) Gas Safe engineer
If the sensor itself tests as healthy but the signal reaching the PCB is still incorrect, the fault lies in the wiring between the two. The engineer will replace the affected section of harness or the full harness as appropriate, then retest.
- Assess the PCB if the fault persists (engineer) Gas Safe engineer
Should the sensor and wiring both prove to be in good condition, the engineer will investigate the PCB as the likely remaining cause. PCB diagnosis may involve substitution testing or specialist analysis. Replacement is a more involved and costly job, so a competent engineer will exhaust all other possibilities first.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has not returned to normal operation after your reset attempt, contact a Gas Safe registered engineer. All diagnostic and repair work on internal boiler components must be carried out by someone who holds current Gas Safe registration — it is a legal requirement in the UK, and attempting this work yourself would invalidate your warranty and home insurance.
Parts you may need
- Flow NTC thermistor (heating flow sensor) · from £25
- Wiring harness / sensor loom · from £45
- 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 the Protherm F.71 fault dangerous?
F.71 itself is not a gas-leak or combustion fault, but the boiler locks out as a precaution because it cannot reliably control the flow temperature without a working sensor. You should not attempt to bypass or repeatedly reset the boiler to keep it running. Leave the heating off and arrange for a Gas Safe engineer to diagnose and fix the fault before you use the boiler again.
Can I fix the Protherm F.71 fault myself?
The only safe DIY action is a single boiler reset, which occasionally clears a transient fault. All other work — checking sensor resistance, repositioning or replacing the NTC thermistor, inspecting the wiring harness, or assessing the PCB — must be done by a Gas Safe registered engineer. Opening the boiler casing and working on its electrical or gas components without the correct registration is illegal and will void your warranty and home insurance.
How much does it cost to fix the Protherm F.71 fault in the UK?
For the majority of cases — where the repair involves replacing the flow NTC thermistor and the labour to diagnose and fit it — you can expect to pay roughly £100 to £280 in total, including parts and a Gas Safe engineer's callout fee. The sensor itself typically costs £15–£40. If the investigation reveals the fault lies with the PCB rather than the sensor, the PCB alone can cost £180–£400 or more for the part, pushing the total bill higher — so it is worth getting a clear diagnosis before authorising an expensive PCB replacement.
Why does my Protherm keep showing F.71 even after a reset?
A reset clears the fault flag but does nothing to fix the underlying problem. If F.71 returns each time you restart the boiler, it means the flow NTC sensor (or its wiring) is consistently sending a fixed, unchanging value to the PCB. Common reasons for this include a failed sensor, a sensor that has become dislodged from its mounting, or limescale build-up coating the sensor in a hard-water area. Each of these needs a physical repair by a Gas Safe engineer — repeated resets will not resolve the fault.