F.00 Boiler Fault Code: What It Means on Every Brand

The F.00 fault code appears on 4 boiler brands — Heatline, Protherm, Saunier Duval and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 4 brands use this code

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.00 means	Severity	DIY?
HHeatlineshown as F00	The F00 code appears when your Heatline boiler's control board (PCB) cannot receive a valid temperature reading from the NTC thermistor fitted to the heating flow pipe. Full guide →	Lockout — engineer likely	No
PProtherm	The F.00 code appears on Protherm boilers when the electronic control unit stops receiving a valid signal from the heating flow temperature sensor, known as NTC2. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.00 fault code (sometimes displayed as F0) appears on Saunier Duval boilers when the control board loses its signal from the flow temperature NTC thermistor. Full guide →	Lockout — engineer likely	No
VVaillant	The F.00 fault code (sometimes shown as F.0 on older displays) tells you that the boiler's heating-flow NTC thermistor has an open circuit — in plain terms, the sensor that monitors the… Full guide →	Lockout — engineer likely	No

F.00 brand by brand

Heatline F00

The F00 code appears when your Heatline boiler's control board (PCB) cannot receive a valid temperature reading from the NTC thermistor fitted to the heating flow pipe. This sensor tells the boiler how hot the water is as it heads out to your radiators. When the PCB detects an open circuit — meaning no resistance signal is getting through — it locks out and displays F00 to flag the problem. Because the boiler is flying blind on flow temperature, it shuts down as a safety precaution.

Causes, fixes & costs for Heatline F00 → · All Heatline codes

Protherm F.00

The F.00 code appears on Protherm boilers when the electronic control unit stops receiving a valid signal from the heating flow temperature sensor, known as NTC2. This sensor sits on the heating flow pipe and continuously feeds temperature data back to the boiler's PCB. When that signal disappears — whether because the sensor has failed, its wiring has come loose, or the circuit is broken — the boiler treats it as a safety-critical condition and shuts down ignition completely, leaving you with no heating or hot water. On the display, the letter 'F' will flash alongside the code '00'. Related codes F.01, F.10, and F.11 follow the same diagnostic logic but target different sensors (return and secondary circuits); if you see one of those instead, the approach described here still applies.

Causes, fixes & costs for Protherm F.00 → · All Protherm codes

Saunier Duval F.00

The F.00 fault code (sometimes displayed as F0) appears on Saunier Duval boilers when the control board loses its signal from the flow temperature NTC thermistor. This sensor continuously monitors the temperature of water leaving the heat exchanger, feeding real-time data back to the PCB so the boiler can regulate the burner output. When the board detects no signal — or a signal so far outside the expected range that it cannot be trusted — it shuts the boiler down and displays F.00. Both central heating and hot water will be unavailable until the fault is resolved. The fault can point to the sensor itself, the wiring between the sensor and the PCB, or a loose plug connection at either end. A healthy flow NTC thermistor reads approximately 10 kΩ resistance at 25°C; as water temperature rises, resistance falls — so a reading that stays fixed or is wildly out of range confirms a faulty component.

Causes, fixes & costs for Saunier Duval F.00 → · All Saunier Duval codes

Vaillant F.00

The F.00 fault code (sometimes shown as F.0 on older displays) tells you that the boiler's heating-flow NTC thermistor has an open circuit — in plain terms, the sensor that monitors the temperature of water leaving the heat exchanger has lost its connection to the control board. Because the boiler can no longer safely track flow temperature, it locks out as a precaution and will not fire until the issue is resolved. The fault sits firmly in electrical/sensor territory: the physical sensor itself, the wiring between it and the PCB, or in rarer cases the PCB itself is responsible.

Causes, fixes & costs for Vaillant F.00 → · All Vaillant codes

Frequently asked questions

Is the F.00 fault code the same on every boiler?

Broadly, yes — on Heatline, Protherm, Saunier Duval and Vaillant the F.00 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F.00 fault myself?

Not usually. On every brand listed, F.00 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.00 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.