F.03 Boiler Fault Code: What It Means on Every Brand

The F.03 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.03 means	Severity	DIY?
PProtherm	The F.03 code on a Protherm boiler has two distinct meanings depending on which model range you own. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.03 code on Saunier Duval boilers has two distinct meanings depending on which model series you own. Full guide →	Lockout — engineer likely	No
VVaillant	The F.03 code on a Vaillant boiler signals that the boiler's control unit has lost a valid reading from a key temperature sensor — triggering a safety lock-out and stopping the boiler from… Full guide →	Lockout — engineer likely	No

F.03 brand by brand

Protherm F.03

The F.03 code on a Protherm boiler has two distinct meanings depending on which model range you own. On Gepard and Panther series boilers, F.03 points to a problem with an NTC temperature sensor — typically the boiler body or cylinder sensor — where the control board is receiving no signal, an out-of-range reading, or data it cannot trust. On Lynx and Leopard series boilers, F.03 indicates that the high-temperature safety thermostat has tripped, meaning water inside the boiler reached approximately 103°C and the overheat protection cut in. In both cases the letter F will flash on the display alongside the code number. Whichever variant applies to your boiler, a reset can be tried first, but if the code returns you will need a Gas Safe registered engineer to investigate further.

Causes, fixes & costs for Protherm F.03 → · All Protherm codes

Saunier Duval F.03

The F.03 code on Saunier Duval boilers has two distinct meanings depending on which model series you own. On older Thema and ThemaClassic series boilers, F.03 signals a repetitive air or fan fault — the boiler previously detected an airflow problem (shown as F.02), attempted to recover automatically, and has now failed enough times that it has escalated into a hard lockout. On newer condensing models, F.03 can instead point to an interrupted or disconnected cylinder or heat-start temperature sensor (NTC), caused by a faulty sensor, damaged wiring, or a loose plug connection. Check your model's manual to confirm which applies to your boiler, but the diagnostic and safety steps below cover both scenarios.

Causes, fixes & costs for Saunier Duval F.03 → · All Saunier Duval codes

Vaillant F.03

The F.03 code on a Vaillant boiler signals that the boiler's control unit has lost a valid reading from a key temperature sensor — triggering a safety lock-out and stopping the boiler from firing. On most current Vaillant models (particularly those paired with an actoSTOR unvented cylinder), F.03 points to a fault with the cylinder NTC sensor or its wiring harness. The boiler relies on this sensor to monitor hot-water cylinder temperature; without a reliable signal, it cannot operate safely. On some older Vaillant VU and VUW series models, F.03 instead relates to the flue gas temperature sensor — the component that monitors the temperature of exhaust gases leaving the boiler. If that sensor detects an excessively high flue temperature or stops sending a reading altogether, the boiler shuts down to prevent unsafe operation. Check your model's documentation or the engineer menu to confirm which sensor applies to your boiler before any diagnostic work begins.

Causes, fixes & costs for Vaillant F.03 → · All Vaillant codes

Frequently asked questions

Is the F.03 fault code the same on every boiler?

Broadly, yes — on Protherm, Saunier Duval and Vaillant the F.03 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.03 fault myself?

Not usually. On every brand listed, F.03 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.03 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.