F12 Boiler Fault Code: What It Means on Every Brand

The F12 fault code appears on 5 brands — Ferroli, Glow-worm, 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 F12 means	Severity	DIY?
FFerroli	The F12 code on a Ferroli boiler indicates a problem with the boiler compartment temperature sensor — typically an NTC (negative temperature coefficient) sensor positioned within the… Full guide →	Engineer may be needed	No
GWGlow-worm	The F12 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) and the display PCB have lost the ability to communicate reliably with each other. Full guide →	Lockout — engineer likely	No
PProthermshown as F.12	The F.12 fault code on most Protherm boilers indicates that the domestic hot water (DHW) temperature sensor has developed a short circuit or is sending a signal that falls outside the… Full guide →	Engineer may be needed	No
SDSaunier Duval	The F12 code on a Saunier Duval boiler signals a breakdown in electronic communication within the boiler's control system. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.12	The F.12 code appears on Vaillant boilers when the appliance detects a blockage that is directly affecting system pressure. Full guide →	Lockout — engineer likely	Some checks

F12 brand by brand

Ferroli F12

The F12 code on a Ferroli boiler indicates a problem with the boiler compartment temperature sensor — typically an NTC (negative temperature coefficient) sensor positioned within the heating circuit or heat exchanger assembly. The sensor is either producing readings that fall outside the expected range, has developed a poor electrical connection, or has failed outright. Because F12 is an "F" prefix blocking fault rather than an "A" prefix lockout, the boiler may recover and clear the code by itself once the underlying condition is resolved — though if it keeps returning, an engineer visit is needed. Note that sensor code behaviour can vary slightly across Ferroli ranges including the Modena, Bluehelix, Optimax, and i-Series, so always cross-check with your specific model's manual if in doubt.

Causes, fixes & costs for Ferroli F12 → · All Ferroli codes

Glow-worm F12

The F12 code on a Glow-Worm boiler signals that the main printed circuit board (PCB) and the display PCB have lost the ability to communicate reliably with each other. When this link breaks down, the boiler's control system can no longer function correctly, so it shuts itself down as a precaution. You may notice the display flickering or going blank, the controls becoming unresponsive, and a complete loss of heating and hot water. This is a lockout fault — the boiler will not restart on its own until the underlying communication problem is identified and corrected.

Causes, fixes & costs for Glow-worm F12 → · All Glow-worm codes

Protherm F.12

The F.12 fault code on most Protherm boilers indicates that the domestic hot water (DHW) temperature sensor has developed a short circuit or is sending a signal that falls outside the acceptable range. The boiler's control unit continuously monitors readings from several sensors; when the DHW sensor signal is missing, erratic, or looks unreliable, the unit logs F.12 and shuts down to protect the system. On the display, the letter F flashes alongside the code 12. Important model note: on older Protherm Lynx Combi models, F.12 carries a different meaning — a low voltage fault on the electrical supply — rather than a sensor issue. Always check your specific model's documentation to confirm the interpretation before proceeding.

Causes, fixes & costs for Protherm F.12 → · All Protherm codes

Saunier Duval F12

The F12 code on a Saunier Duval boiler signals a breakdown in electronic communication within the boiler's control system. Depending on the model, this can mean the user interface board is not talking correctly to the main PCB, there is a fault on the main board itself, or — on certain low-temperature models — a short circuit or voltage problem on the eBus communication line between internal components. In all cases the boiler has detected that data is no longer flowing reliably between its electronic boards and has shut down as a precaution. The boiler typically stops responding to thermostat commands and will not fire until the fault is resolved.

Causes, fixes & costs for Saunier Duval F12 → · All Saunier Duval codes

Vaillant F.12

The F.12 code appears on Vaillant boilers when the appliance detects a blockage that is directly affecting system pressure. The boiler locks out as a safety measure and will not restart until the root cause is identified and resolved. This code sits between the related F.11 (blockage preventing circulation flow) and F.13 (blockage caused by insufficient water), but F.12 specifically points to a pressure-related obstruction within the appliance or heating circuit. Because the fault can involve how the boiler interacts with the wider heating system, a Gas Safe registered engineer is usually needed to carry out a thorough diagnosis.

Causes, fixes & costs for Vaillant F.12 → · All Vaillant codes

Frequently asked questions

Is the F12 fault code the same on every boiler?

No. F12 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 F12 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F12 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.