F.71 Boiler Fault Code: What It Means on Every Brand

The F.71 fault code appears on 5 brands — 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.

Up to: Lockout — engineer likely 5 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F.71 means	Severity	DIY?
GWGlow-wormshown as F71	The F71 code appears on Glow-Worm boilers when the control board detects a problem with one or both of the water temperature sensors — specifically the flow sensor (which monitors water… Full guide →	Lockout — engineer likely	No
HHeatlineshown as F71	The F71 code on Heatline boilers indicates a fault with the flow heating temperature sensor (NTC thermistor). Full guide →	Engineer may be needed	No
PProtherm	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. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.71 code appears when your Saunier Duval boiler detects that the flow temperature sensor (NTC) is reporting a fixed, unchanging value instead of the live temperature readings the… Full guide →	Lockout — engineer likely	No
VVaillant	The F.71 code appears on Vaillant boilers when the PCB detects a problem with the heating flow NTC thermistor — the small temperature sensor fitted to the flow pipe (the pipe carrying hot… Full guide →	Lockout — engineer likely	No

F.71 brand by brand

Glow-worm F71

The F71 code appears on Glow-Worm boilers when the control board detects a problem with one or both of the water temperature sensors — specifically the flow sensor (which monitors water leaving the boiler) or the return sensor (which monitors cooler water coming back from the heating circuit). These sensors are NTC thermistors: small resistive components whose electrical resistance changes as temperature rises or falls, giving the PCB a continuous read-out of water temperature. When the PCB receives a reading that is implausible, out of range, or missing entirely, it triggers F71 and shuts the boiler down as a precaution. The fault can point to a wiring or connector issue, a failed thermistor, a circulation problem, or — less commonly — a defective internal expansion vessel or PCB fault affecting how sensor signals are interpreted.

Causes, fixes & costs for Glow-worm F71 → · All Glow-worm codes

Heatline F71

The F71 code on Heatline boilers indicates a fault with the flow heating temperature sensor (NTC thermistor). Specifically, the boiler's control board has detected that the flow sensor reading is stuck — remaining static rather than changing as it should during normal operation. A healthy flow sensor continuously reports shifting temperatures as the boiler fires and water circulates; when those readings flatline, the PCB flags F71 and shuts down to protect the system. The most common trigger is the thermistor becoming physically dislodged from the flow pipe on the heat exchanger, though a failed sensor or wiring fault can produce the same result. Heatline boilers (Capriz, CaprizPlus, Advance, Kalder and related models) share design heritage with Vaillant boilers within the same group, so this code carries the same meaning across both brands. If your display is cycling between F71 and another code every two seconds, the boiler has logged multiple simultaneous faults — note them all down before resetting.

Causes, fixes & costs for Heatline F71 → · All Heatline codes

Protherm F.71

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.

Causes, fixes & costs for Protherm F.71 → · All Protherm codes

Saunier Duval F.71

The F.71 code appears when your Saunier Duval boiler detects that the flow temperature sensor (NTC) is reporting a fixed, unchanging value instead of the live temperature readings the boiler expects. Because the boiler relies on this sensor to monitor the temperature of water leaving the heat exchanger, a stuck or static signal causes the boiler to shut down as a safety precaution. The most common reasons for this are that the sensor has slipped out of position on the flow pipe, or that the sensor itself has failed electrically. In rarer cases, a wiring fault or a problem with the PCB can produce the same symptom. Note that the neighbouring code F.70 relates to an invalid Device Specific Number (DSN) — typically triggered when a display and PCB are replaced simultaneously without the DSN being correctly configured — so if your display is alternating between F.70 and F.71 after recent board work, that distinction is worth flagging to your engineer.

Causes, fixes & costs for Saunier Duval F.71 → · All Saunier Duval codes

Vaillant F.71

The F.71 code appears on Vaillant boilers when the PCB detects a problem with the heating flow NTC thermistor — the small temperature sensor fitted to the flow pipe (the pipe carrying hot water out of the boiler). Your boiler uses two NTC thermistors, one on the flow and one on the return, to continuously monitor water temperature. With F.71 specifically, the PCB has either lost its signal from the flow sensor entirely, or noticed no temperature rise at all when the burner fires up. The result is that the boiler locks out to protect itself. You may have noticed intermittent heating, no hot water, or the boiler cutting out shortly after starting before this lockout occurred.

Causes, fixes & costs for Vaillant F.71 → · All Vaillant codes

Frequently asked questions

Is the F.71 fault code the same on every boiler?

No. F.71 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F.71 fault myself?

Not usually. On every brand listed, F.71 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F.71 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

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.