F2 Boiler Fault Code: What It Means on Every Brand

F2 is a flame-related fault on Ideal, Keston and Viessmann (flame lost or never established), but a combustion-fan fault on Glow-worm and a heating-sensor fault on Protherm. The Ideal version — closely related to L2 — is the most searched, and in winter it is almost always a frozen condensate pipe.

Lockout — engineer likely 5 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 F2 means	Severity	DIY?
GWGlow-worm	The F2 fault code on Glow-Worm boilers indicates a problem with the combustion fan — the component responsible for drawing in fresh air and expelling flue gases safely out of your home. Full guide →	Lockout — engineer likely	No
IIdeal	The F2 fault code on Ideal boilers indicates that the boiler either failed to establish a stable flame during ignition or lost the flame while running. Full guide →	Lockout — engineer likely	No
KKeston	The F2 code on a Keston boiler signals a flame loss event — the boiler attempted to burn but the flame either failed to establish or dropped out during operation. Full guide →	Lockout — engineer likely	No
PProthermshown as F.2	The F.2 code on Protherm boilers signals that the control board has lost a valid reading from the heating circuit flow temperature sensor — typically an NTC thermistor mounted on the flow… Full guide →	Lockout — engineer likely	No
VViessmann	The F2 code on a Viessmann boiler signals a burner lockout — the boiler's built-in safety system has detected a problem and shut down the burner to protect the appliance. Full guide →	Lockout — engineer likely	Some checks

F2 brand by brand

Glow-worm F2

The F2 fault code on Glow-Worm boilers indicates a problem with the combustion fan — the component responsible for drawing in fresh air and expelling flue gases safely out of your home. When the boiler's control board detects that the fan is not spinning at the correct speed, or has stopped working altogether, it triggers the F2 lockout as a safety measure to prevent harmful gases from accumulating. On certain older Glow-Worm models the F2 code can also relate to flame loss during operation, but across the most common current ranges (including the Flexicom, Ultracom, and Energy ranges) a fan-related fault is the primary meaning. Because the fan is critical for safe combustion, the boiler will refuse to attempt ignition until the fault is resolved.

Causes, fixes & costs for Glow-worm F2 → · All Glow-worm codes

Ideal F2

The F2 fault code on Ideal boilers indicates that the boiler either failed to establish a stable flame during ignition or lost the flame while running. As a safety measure, the boiler's PCB shuts the unit down immediately and locks it out — it will not attempt to restart on its own. This is designed to prevent unburned gas from accumulating inside the appliance.

Causes, fixes & costs for Ideal F2 → · All Ideal codes

Keston F2

The F2 code on a Keston boiler signals a flame loss event — the boiler attempted to burn but the flame either failed to establish or dropped out during operation. Depending on how persistent the problem is, the boiler will either pause and retry automatically (a block) or lock out completely and wait for you to press the Reset button. Switching the boiler off and on at the programmer or mains will not clear an F2 lockout; the Reset button on the boiler itself must be used. Keston's twin-flue design means the flue system is a particularly important area to investigate, as dust and spider webs can accumulate in places not seen on conventional boilers.

Causes, fixes & costs for Keston F2 → · All Keston codes

Protherm F.2

The F.2 code on Protherm boilers signals that the control board has lost a valid reading from the heating circuit flow temperature sensor — typically an NTC thermistor mounted on the flow pipe. Without a reliable temperature signal, the boiler cannot safely regulate water temperature, so it shuts down immediately as a precaution. The code appears across the Panther, Gepard, and Leopard ranges (and is consistent with similar older models such as the Lynx Kombi), where it always relates to the heating flow sensor circuit rather than the domestic hot water sensor. On supported models the display will flash 'F' alongside '2'. Note: if you own a different Protherm range and see F.2, always cross-check your installation manual, as Protherm uses a shared fault-code framework across several product families and the label can occasionally differ on older units.

Causes, fixes & costs for Protherm F.2 → · All Protherm codes

Viessmann F2

The F2 code on a Viessmann boiler signals a burner lockout — the boiler's built-in safety system has detected a problem and shut down the burner to protect the appliance. Most commonly, the PCB has identified a circulation fault or pump issue and has prevented the boiler from firing until the underlying cause is resolved. No heat or hot water will be produced while the lockout is active. Once the fault is cleared, the boiler will need a manual reset before it resumes normal operation.

Causes, fixes & costs for Viessmann F2 → · All Viessmann codes

Frequently asked questions

Is the F2 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Ideal, Keston, Protherm and Viessmann the F2 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 F2 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 F2 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.