F.02 Boiler Fault Code: What It Means on Every Brand

The F.02 fault code appears on 3 boiler brands — 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 3 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.02 means	Severity	DIY?
PProtherm	The Protherm F.02 (also displayed as F2) fault code points to a problem with the heating circuit flow NTC thermistor — the temperature sensor that monitors how hot the water is as it… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.02 code (sometimes displayed as F2) signals that your Saunier Duval boiler has detected a problem with its airflow or combustion ventilation circuit. Full guide →	Lockout — engineer likely	No
VVaillant	The F.02 code appears when your Vaillant boiler cannot get a reliable temperature reading from its flow NTC thermistor — the small sensor that continuously monitors the temperature of… Full guide →	Lockout — engineer likely	No

F.02 brand by brand

Protherm F.02

The Protherm F.02 (also displayed as F2) fault code points to a problem with the heating circuit flow NTC thermistor — the temperature sensor that monitors how hot the water is as it leaves the boiler to circulate around your radiators or to your hot water cylinder. When this sensor gives a reading outside expected parameters, produces no reading at all, or its wiring circuit is open or short-circuited, the boiler locks out as a precaution to prevent unsafe operation. Until the fault is resolved, heating and hot water will be unavailable. The code is used across the Protherm range (including Leopard Kombi models) and, given that Protherm sits within the Vaillant Group, the fault logic closely mirrors what Vaillant-family boilers use for flow sensor failures.

Causes, fixes & costs for Protherm F.02 → · All Protherm codes

Saunier Duval F.02

The F.02 code (sometimes displayed as F2) signals that your Saunier Duval boiler has detected a problem with its airflow or combustion ventilation circuit. The boiler's control board monitors the extraction fan and air pressure switch continuously; if it cannot confirm that flue gases are being safely expelled, it triggers a safety lockout to eliminate any risk of carbon monoxide building up inside your home. The boiler may attempt an automatic restart after roughly 15 minutes, but if the underlying fault remains, F.02 will return. While locked out, the boiler will not provide heating or hot water.

Causes, fixes & costs for Saunier Duval F.02 → · All Saunier Duval codes

Vaillant F.02

The F.02 code appears when your Vaillant boiler cannot get a reliable temperature reading from its flow NTC thermistor — the small sensor that continuously monitors the temperature of water leaving the boiler. Without a trustworthy signal from this sensor, the boiler locks itself out as a safety precaution to prevent overheating or damage to the system. The exact sensor involved varies slightly by model: on ecoTEC combi boilers, F.02 points to the main flow (heating circuit) NTC thermistor, while on older Turbomax and VUW combi models it specifically indicates an open-circuit fault on the domestic hot water (DHW) outlet NTC. In both cases the underlying problem is the same — the sensor is either disconnected, has failed electrically, or is sending a signal the PCB cannot interpret.

Causes, fixes & costs for Vaillant F.02 → · All Vaillant codes

Frequently asked questions

Is the F.02 fault code the same on every boiler?

Broadly, yes — on Protherm, Saunier Duval and Vaillant the F.02 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.02 fault myself?

Not usually. On every brand listed, F.02 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.02 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.