F88 Boiler Fault Code: What It Means on Every Brand

The F88 fault code appears on 2 boiler brands — Glow-worm and Vaillant — 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 2 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 F88 means	Severity	DIY?
GWGlow-worm	The F88 code indicates that your Glow-Worm boiler has detected a problem with combustion — either an unstable or undetectable flame signal, or a fault within the gas valve assembly… Full guide →	Lockout — engineer likely	No
VVaillantshown as F.88	The F.88 code means your Vaillant boiler has detected an electrical fault associated with the gas valve. Full guide →	Lockout — engineer likely	No

F88 brand by brand

Glow-worm F88

The F88 code indicates that your Glow-Worm boiler has detected a problem with combustion — either an unstable or undetectable flame signal, or a fault within the gas valve assembly depending on your model. On most Glow-Worm boilers, F88 points to the flame being erratic or failing to hold reliably after ignition. On EasiCom models specifically, it is more closely associated with the gas valve assembly failing to operate correctly. In either case, the boiler locks out as a safety measure and will not restart until the underlying fault is resolved.

Causes, fixes & costs for Glow-worm F88 → · All Glow-worm codes

Vaillant F.88

The F.88 code means your Vaillant boiler has detected an electrical fault associated with the gas valve. The boiler locks out as a safety precaution because it cannot confirm the gas valve is operating correctly. The gas valve is responsible for regulating how much gas reaches the burner — it opens and closes on instruction from the PCB (printed circuit board) in response to demand from your thermostat or hot water controls. When the PCB loses confidence in the valve's electrical signals, it triggers F.88 and shuts the boiler down. This code sits in the same family as F.61 and F.62, all of which relate to gas valve control issues, but F.88 specifically points to an electrical fault in the valve circuit rather than a mechanical or gas-pressure problem.

Causes, fixes & costs for Vaillant F.88 → · All Vaillant codes

Frequently asked questions

Is the F88 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Vaillant the F88 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 F88 fault myself?

Not usually. On every brand listed, F88 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 F88 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.