F3 Boiler Fault Code: What It Means on Every Brand

F3 is a fan-related fault on most brands that use it — Ideal, Keston and Glow-worm all use F3 to say the flue fan is not running or not reaching the required speed — but Viessmann uses it for a flame-detection problem, and Protherm's meaning depends on the model. Fan faults are engineer territory on every brand; the homeowner-side check is simply whether anything is blocking the flue terminal outside.

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 F3 means	Severity	DIY?
GWGlow-worm	The F3 code means your Glow-Worm boiler has detected that its fan is not spinning fast enough to reach the speed required for safe ignition. Full guide →	Lockout — engineer likely	No
IIdeal	The F3 code on an Ideal boiler signals a problem with the flue fan. Full guide →	Lockout — engineer likely	No
KKeston	The F3 fault code on a Keston boiler indicates a problem with the flue fan. Full guide →	Lockout — engineer likely	No
PProtherm	The Protherm F3 fault code has two distinct meanings depending on your specific model. Full guide →	Engineer may be needed	No
VViessmann	The F3 fault code on Viessmann boilers points to a problem with flame establishment or flame detection during burner start-up. Full guide →	Lockout — engineer likely	No

F3 brand by brand

Glow-worm F3

The F3 code means your Glow-Worm boiler has detected that its fan is not spinning fast enough to reach the speed required for safe ignition. Before the burner is allowed to fire, the boiler's controls verify that the fan is moving at the correct rate to safely carry combustion gases out through the flue. If that target speed isn't achieved within the allowed time, the boiler locks out and displays F3 as a protective measure. Because the fan is responsible for expelling potentially harmful gases — including carbon monoxide — this lockout is a deliberate safety response, not a nuisance trip.

Causes, fixes & costs for Glow-worm F3 → · All Glow-worm codes

Ideal F3

The F3 code on an Ideal boiler signals a problem with the flue fan. The fan's job is to push combustion gases safely out through the flue — without it working correctly, harmful gases could accumulate inside the appliance. When the boiler's PCB detects that the fan is not running at the expected speed, or receives no feedback signal from it at all, it triggers a lockout and displays F3. Both central heating and hot water are suspended until the fault is resolved, because the fan is essential to safe operation in every mode. F3 is a standardised code found across the current Ideal Logic and Vogue ranges; it is not present on older Ideal models.

Causes, fixes & costs for Ideal F3 → · All Ideal codes

Keston F3

The F3 fault code on a Keston boiler indicates a problem with the flue fan. Specifically, the boiler's PCB has detected that the fan is either not running at all or is failing to reach the speed required for safe operation. Because the fan is responsible for drawing combustion gases out through the flue, the boiler will not attempt to ignite without confirming the fan is working correctly — so when F3 appears, the boiler enters lockout and shuts down entirely. Note that the F3 code is associated primarily with the Keston S30 system boiler; if you have a Keston C36 combi, a similar fan-related fault will appear as E05 rather than F3.

Causes, fixes & costs for Keston F3 → · All Keston codes

Protherm F3

The Protherm F3 fault code has two distinct meanings depending on your specific model. On the most widely sold Protherm models in the UK — including the Panther and Leopard ranges — F3 indicates that the boiler has overheated. The safety automation triggers a shutdown when the protective sensor reaches around 103°C, protecting the heat exchanger from damage. The boiler may restart itself once temperatures fall back into the normal operating range, but a recurring F3 strongly suggests an underlying fault. On some other Protherm models (such as the Gepard range), F3 can instead point to the boiler's ventilator or fan failing to maintain the correct rotational speed during operation. Because F3 can mean different things across the Protherm range, always check your model's manual or consult a Gas Safe registered engineer if you are unsure which interpretation applies to your boiler.

Causes, fixes & costs for Protherm F3 → · All Protherm codes

Viessmann F3

The F3 fault code on Viessmann boilers points to a problem with flame establishment or flame detection during burner start-up. In most cases — particularly on the popular Vitodens range — it means the boiler has attempted to light the burner but cannot confirm a flame is present, either because no flame was actually produced or because the signal from the ionisation circuit is absent or too weak to be accepted. On certain models such as the Vitopend, F3 can carry the specific meaning of 'burner on damage', indicating that a flame signal was already present before start-up, suggesting residual ionisation or a stuck-open gas valve. Either way, the boiler locks out as a safety precaution and will not restart automatically. A manual reset is required, but if the underlying fault is not resolved the code will return.

Causes, fixes & costs for Viessmann F3 → · All Viessmann codes

Frequently asked questions

Is the F3 fault code the same on every boiler?

No. F3 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 F3 fault myself?

Not usually. On every brand listed, F3 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 F3 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.