F1 Boiler Fault Code: What It Means on Every Brand

F1 is a trap: six brands use it for six different faults. On Ideal and Keston it is low water pressure — a filling-loop top-up. On Worcester Bosch and Glow-worm it is ignition failure. On Viessmann it is a flue-gas temperature fault, and on Protherm its meaning depends on the model. Anyone who tells you what 'F1 on a boiler' means without asking the brand is guessing — check the table.

Up to: Lockout — engineer likely 6 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 F1 means	Severity	DIY?
GWGlow-worm	The F1 code appears on Glow Worm boilers when the boiler has attempted to light itself five times in a row without the PCB detecting a stable flame. Full guide →	Lockout — engineer likely	Some checks
IIdeal	On Ideal boilers, F1 means the appliance has detected low water pressure in the heating system and has shut itself down to prevent damage. Full guide →	Engineer may be needed	Some checks
KKeston	The F1 code on a Keston S30 system boiler means the boiler has detected that the water pressure in your central heating system has dropped too low — typically below around 0.5 bar. Full guide →	Lockout — engineer likely	Some checks
PProthermshown as F.1	The Protherm F.1 fault code indicates a safety lockout, but crucially its precise meaning depends on which Protherm boiler you own. Full guide →	Lockout — engineer likely	No
VViessmann	The F1 fault code on a Viessmann boiler indicates that the flue gas temperature sensor has detected temperatures beyond the safe operating threshold. Full guide →	Lockout — engineer likely	Some checks
WBWorcester Bosch	The F1 fault code on a Worcester Bosch boiler signals an ignition failure — the boiler has tried to light and been unable to establish a flame. Full guide →	Lockout — engineer likely	No

F1 brand by brand

Glow-worm F1

The F1 code appears on Glow Worm boilers when the boiler has attempted to light itself five times in a row without the PCB detecting a stable flame. After these repeated failed attempts, the boiler locks out completely — cutting off both heating and hot water — as a built-in safety measure to prevent dangerous gas accumulation or incomplete combustion. Because the boiler's display doesn't pinpoint which specific stage of the ignition sequence failed, diagnosing the root cause requires a methodical process of elimination.

Causes, fixes & costs for Glow-worm F1 → · All Glow-worm codes

Ideal F1

On Ideal boilers, F1 means the appliance has detected low water pressure in the heating system and has shut itself down to prevent damage. It usually appears when the system pressure falls below around 1.0 bar. Most of the time it's a straightforward fix — simply topping the pressure back up — but if F1 keeps returning there is an underlying cause, such as a leak or a faulty component, that needs proper investigation.

Causes, fixes & costs for Ideal F1 → · All Ideal codes

Keston F1

The F1 code on a Keston S30 system boiler means the boiler has detected that the water pressure in your central heating system has dropped too low — typically below around 0.5 bar. A pressure sensor inside the boiler monitors system pressure continuously, and when it falls beneath the safe threshold, the boiler shuts itself down to prevent the heat exchanger running dry and being damaged. Once the fault is cleared and the pressure is restored, the boiler must be manually reset before it will fire up again. Note that other Keston models (such as the C40) use a different code — E44 — for the same underlying low-pressure condition; if you own a C40 and see E44, the diagnosis and fixes described here apply equally.

Causes, fixes & costs for Keston F1 → · All Keston codes

Protherm F.1

The Protherm F.1 fault code indicates a safety lockout, but crucially its precise meaning depends on which Protherm boiler you own. On the Protherm Leopard combi — the most commonly installed Protherm model in the UK — F.1 means the boiler has failed to establish or maintain a flame during the ignition sequence. On the Protherm Lynx combi and the Protherm 80E (sometimes sold as a re-badged Glow-worm or Ideal unit), F.1 signals an overheat safety lockout, meaning the system detected water temperatures exceeding safe limits and shut itself down. In both cases the boiler stops producing heat or hot water and will not restart until the underlying fault is resolved. Because the root cause differs between these two families, it is important to identify your exact model before following any diagnostic steps.

Causes, fixes & costs for Protherm F.1 → · All Protherm codes

Viessmann F1

The F1 fault code on a Viessmann boiler indicates that the flue gas temperature sensor has detected temperatures beyond the safe operating threshold. This triggers an automatic safety lockout, shutting the boiler down to prevent damage. Despite the flue-related description, the root cause is almost always a circulation or pressure problem rather than an actual fault with the flue itself — something is preventing heat from being carried away from the boiler efficiently, allowing temperatures to climb dangerously high. A note on model variation: on certain older Viessmann models, some third-party sources associate F1 with a return temperature sensor fault rather than flue gas overtemperature — always cross-reference with your specific model's documentation if in doubt.

Causes, fixes & costs for Viessmann F1 → · All Viessmann codes

Worcester Bosch F1

The F1 fault code on a Worcester Bosch boiler signals an ignition failure — the boiler has tried to light and been unable to establish a flame. This puts the boiler into a locking fault (lockout), shown on the display as a flashing triangle alongside the static F1 code. Once the underlying problem is resolved, the boiler must be manually reset to clear the code and resume normal operation. Important: Do not confuse this with the F1 code on Ideal boilers, where F1 means low water pressure. On Worcester Bosch, F1 is always ignition-related — not a pressure fault. You may also see the code displayed with a numeric sub-code, for example F1 259 or F1 263. These give additional detail about where the fault lies: F1 259 points to a defect in the control box or Heat Control Module (HCM), often related to connections between the two. F1 263 similarly indicates a control box or HCM issue, but specifically suggests the HCM may have become dislodged from the boiler, or that an incompatible control box has been fitted. Both sub-codes require an engineer to investigate.

Causes, fixes & costs for Worcester Bosch F1 → · All Worcester Bosch codes

Frequently asked questions

Is the F1 fault code the same on every boiler?

No. F1 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 F1 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a F1 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.