F4 Boiler Fault Code: What It Means on Every Brand

F4 has three distinct meanings. On Glow-worm and Viessmann it is a flame-detection failure — the boiler could not confirm the burner lit. On Keston it is the flow thermistor, and on Ideal it is a sensor fault that also appears as L4 on some models. The Viessmann version is the most searched, largely because 'F4' is how older Vitodens displays render what other documentation calls E4.

Lockout — engineer likely 4 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 F4 means	Severity	DIY?
GWGlow-worm	The F4 code means your Glow-Worm boiler has failed to detect a flame during the ignition sequence. Full guide →	Lockout — engineer likely	No
IIdeal	The F4 code — also displayed as L4 on certain Ideal models — indicates that the boiler has detected a problem with its flow thermistor, the small temperature sensor that monitors how hot… Full guide →	Lockout — engineer likely	No
KKeston	The F4 fault code on Keston boilers indicates a problem with the flow thermistor — the NTC temperature sensor that monitors the temperature of water leaving the heat exchanger. Full guide →	Lockout — engineer likely	No
VViessmann	The F4 fault code on a Viessmann boiler means the control system has been unable to confirm that the burner lit successfully during an ignition attempt. Full guide →	Lockout — engineer likely	No

F4 brand by brand

Glow-worm F4

The F4 code means your Glow-Worm boiler has failed to detect a flame during the ignition sequence. When the burner either doesn't light at all or the flame sensor can't confirm combustion has taken place, the boiler's PCB triggers a safety lockout and displays F4. This prevents unburnt gas from building up inside the appliance — a genuine safety concern. Until the root cause is found and resolved, the boiler will refuse to fire. A single reset may clear a one-off hiccup, but if the fault keeps returning, something physical needs attention. Most of the underlying causes require a Gas Safe registered engineer to diagnose and repair safely.

Causes, fixes & costs for Glow-worm F4 → · All Glow-worm codes

Ideal F4

The F4 code — also displayed as L4 on certain Ideal models — indicates that the boiler has detected a problem with its flow thermistor, the small temperature sensor that monitors how hot the water is as it leaves the heat exchanger. Because the boiler can no longer confirm it is operating within safe temperature limits, it shuts down as a safety precaution and will not restart until the underlying issue is resolved. This is a hard lockout, meaning a manual reset is required before the boiler will attempt to fire again.

Causes, fixes & costs for Ideal F4 → · All Ideal codes

Keston F4

The F4 fault code on Keston boilers indicates a problem with the flow thermistor — the NTC temperature sensor that monitors the temperature of water leaving the heat exchanger. When this sensor sends a reading outside the expected range, or stops responding altogether, the boiler interprets it as a potential safety risk and shuts down. F4 triggers a full lockout, meaning the boiler will not attempt to restart until the reset button is pressed. A closely related code, L4, indicates the same underlying fault but results in a temporary block rather than a full lockout — the boiler pauses for a set period before trying to re-fire on its own. If you see L4 repeatedly, it is likely to escalate to F4. Keston boilers (including the S30, C36, C40, and C55) are considered more complex appliances than standard combi boilers due to their twin-flue design and modulating pump, so diagnosis and repair should ideally be carried out by an engineer with specific Keston experience.

Causes, fixes & costs for Keston F4 → · All Keston codes

Viessmann F4

The F4 fault code on a Viessmann boiler means the control system has been unable to confirm that the burner lit successfully during an ignition attempt. The boiler's ionisation probe (flame sensor) is expected to detect a stable flame current shortly after the ignition sequence begins. If no flame signal is received within the allotted time — or the signal is lost mid-run — the boiler locks out and displays F4 as a safety measure. Both central heating and hot water are unavailable while the fault is active. In some cases, you may notice the boiler attempting to fire, hear clicking from the igniter, and then go quiet before the code appears. Intermittent heating performance in the days before a full lockout is also a common warning sign that the flame signal has been struggling.

Causes, fixes & costs for Viessmann F4 → · All Viessmann codes

Frequently asked questions

Is the F4 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Ideal, Keston and Viessmann the F4 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 F4 fault myself?

Not usually. On every brand listed, F4 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 F4 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.