F8 Boiler Fault Code: What It Means on Every Brand

The F8 fault code appears on 3 boiler brands — Glow-worm, Ideal and Viessmann — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 3 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 F8 means	Severity	DIY?
GWGlow-worm	The F8 code on a Glow-Worm boiler signals a problem with the tank thermistor — an NTC (Negative Temperature Coefficient) sensor that monitors the temperature of stored domestic hot water… Full guide →	Lockout — engineer likely	No
IIdeal	The F8 fault code on Ideal boilers signals a problem with the Printed Circuit Board (PCB) — either the board itself has developed a fault, or a replacement PCB has been fitted but not… Full guide →	Lockout — engineer likely	No
VViessmann	The F8 fault code on a Viessmann boiler means the burner control has entered lockout because the fan was still spinning when the boiler expected it to be completely at rest — or because a… Full guide →	Lockout — engineer likely	No

F8 brand by brand

Glow-worm F8

The F8 code on a Glow-Worm boiler signals a problem with the tank thermistor — an NTC (Negative Temperature Coefficient) sensor that monitors the temperature of stored domestic hot water in a cylinder or tank. This sensor is primarily relevant to system boilers and heat-only boilers paired with a hot water storage cylinder. When the boiler's control board receives a reading from this sensor that falls outside its expected range — or receives no reading at all — it treats the situation as a fault and locks the boiler out to prevent damage. The F8 code is confirmed on older Glow-Worm ranges including the Ultracom, Flexicom, Xtramax, and Energy series; always verify against your specific model's documentation as fault code definitions can vary between product generations.

Causes, fixes & costs for Glow-worm F8 → · All Glow-worm codes

Ideal F8

The F8 fault code on Ideal boilers signals a problem with the Printed Circuit Board (PCB) — either the board itself has developed a fault, or a replacement PCB has been fitted but not correctly configured for the specific boiler model. You may also see this displayed as L8 on some Ideal models; the L prefix typically indicates a lockout rather than a simple fault, but both point to the same underlying PCB issue. When the boiler detects this fault it enters a safety lockout, shutting down completely until the problem is investigated and resolved. This is one of the more serious fault codes Ideal boilers can display, and in most cases a Gas Safe registered engineer will be needed to diagnose and fix it.

Causes, fixes & costs for Ideal F8 → · All Ideal codes

Viessmann F8

The F8 fault code on a Viessmann boiler means the burner control has entered lockout because the fan was still spinning when the boiler expected it to be completely at rest — or because a plug-in connection between the fan and the control board is unstable. During a restart or auto-reset sequence, the PCB checks that the fan RPM has dropped to zero before allowing the burner to fire. If the fan is still turning (even slowly) or if the wiring connection is unreliable, the boiler treats this as an unsafe condition and shuts down. The code can appear across several Viessmann ranges — including the Vitodens and Vitopend series — though the precise trigger can differ slightly between models, so always cross-reference with your boiler's own manual or have an engineer pull the full fault history.

Causes, fixes & costs for Viessmann F8 → · All Viessmann codes

Frequently asked questions

Is the F8 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Ideal and Viessmann the F8 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 F8 fault myself?

Not usually. On every brand listed, F8 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 F8 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.