The F10 fault code appears on 6 brands — Ferroli, Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F10 brand by brand
The F10 code on a Ferroli boiler indicates a fault with the central heating flow temperature sensor (also referred to as the delivery or supply sensor). This NTC thermistor monitors the temperature of water leaving the heat exchanger on its way around your heating circuit. When the boiler's control board detects that the sensor's signal is outside acceptable parameters — whether due to an open circuit, a short circuit, or physical sensor damage — it throws F10 and shuts the boiler down entirely. Unlike the related F11 code (which affects the domestic hot water sensor and allows heating to continue), F10 causes a full stop: neither heating nor hot water will work until the fault is resolved. Because F10 is an 'F' category code on Ferroli boilers, it is classified as a blocking fault that may clear on its own if the underlying condition resolves, but in practice a persistent F10 almost always requires a physical repair.
Causes, fixes & costs for Ferroli F10 → · All Ferroli codes
The F10 code appears when your Glow Worm boiler detects a problem with either the flow heating temperature sensor (NTC thermistor) or, on certain models, the flame detection circuit. In the majority of cases — particularly on popular ranges such as the Betacom4 and Ultracom3 — the fault points to the flow NTC thermistor, a small electrical component that continuously feeds water temperature data back to the PCB. When this sensor fails, reads out of range, or loses its connection, the boiler cannot safely regulate heating output and shuts down. On some older Glow Worm models, F10 instead flags a problem with the flame-sensing electrode, meaning the boiler cannot confirm a stable flame is present. In either case, the boiler will typically lock out and refuse to fire until the fault is resolved.
Causes, fixes & costs for Glow-worm F10 → · All Glow-worm codes
The F10 code appears on Heatline boilers when the control board detects a problem with the flow heating temperature sensor — the NTC thermistor that monitors water temperature as it leaves the heat exchanger. Specifically, the board is seeing a reading that suggests a short circuit or abnormally low resistance in the sensor circuit, which the boiler interprets as an unreliable or corrupted signal. Because accurate flow temperature data is essential for safe burner operation, the boiler shuts down and locks out until the fault is resolved. A healthy flow NTC should read around 12 kΩ at 20°C; a significantly lower reading points to a shorted sensor or damaged wiring. Note that Heatline is part of the Vaillant Group and its boilers share diagnostic logic with Vaillant appliances, so F10 carries the same meaning across both brands.
Causes, fixes & costs for Heatline F10 → · All Heatline codes
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.
Causes, fixes & costs for Protherm F.10 → · All Protherm codes
The F10 code indicates that the boiler's central heating return thermistor (NTC sensor) has developed a fault. This small temperature sensor sits on the return pipework and continuously reports the temperature of water coming back into the boiler from the heating circuit. When the boiler's control board detects that the sensor's signal is missing, out of range, or short-circuited, it triggers F10 and shuts the boiler down to prevent unsafe operation.
Causes, fixes & costs for Saunier Duval F10 → · All Saunier Duval codes
The F.10 code on a Vaillant boiler indicates that the heating flow NTC thermistor — the sensor that monitors the temperature of water leaving the heat exchanger on its way out to your radiators — has reported a short circuit. Because the boiler cannot obtain a reliable flow temperature reading, it locks out as a safety precaution. This is the flow (outgoing) sensor, not the return sensor. The fault will not clear permanently until the underlying electrical or sensor issue is resolved; a reset may temporarily clear the display, but the code will almost certainly return if the root cause remains.
Causes, fixes & costs for Vaillant F.10 → · All Vaillant codes
General guidance only — not a substitute for professional advice, and not certified by a registered engineer.
Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
