Glow-worm F88 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F88 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Dirty or faulty flame sensor Common
The flame sensor (also called the flame rectification electrode) monitors the presence of a stable flame inside the burner. Carbon deposits or general wear can prevent it from reading the flame accurately, causing the boiler to believe the flame has failed even when it has briefly lit. This is one of the most frequent reasons for an F88 lockout.
- Faulty or failing gas valve Common
The gas valve controls exactly how much gas enters the burner. On EasiCom models this is the primary cause flagged by F88. A valve that is sticking, not opening fully, or failing to close correctly will produce an erratic or weak flame — or trigger an internal fault signal within the valve assembly itself.
- Low or unstable gas supply pressure Sometimes
If the gas pressure reaching the boiler is below the manufacturer's specification, the burner cannot sustain a consistent flame. This can stem from a problem at the meter, an issue with the gas network, or a partially closed isolation valve. Other gas appliances in the home may also perform poorly if this is the cause.
- Wiring or electrical connection fault Sometimes
Damaged, loose, or corroded wiring between the PCB, gas valve, and flame sensor can send incorrect signals that mimic a flame or valve failure. Vibration over time can loosen terminals, and moisture ingress can cause intermittent faults.
- PCB fault Rare
The printed circuit board manages every electrical signal in the boiler. If the PCB is sending incorrect commands to the gas valve or misreading the flame sensor signal, it can generate an F88 lockout even when the combustion components themselves are in reasonable condition. PCB faults are less common but should be considered if other components test correctly.
How to fix it
- Check that your gas supply is active DIY safe
Confirm the gas meter isolation valve is fully open and that other gas appliances in your home — a gas hob, for example — are working normally. If nothing gas-powered is working, contact your gas supplier rather than an engineer, as the fault may lie with the supply network.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or 'R' symbol) for around 3 seconds, then release it and allow the boiler up to 2 minutes to attempt a restart. If it fires up and runs normally, monitor it closely over the next few hours. If F88 returns, do not reset again — repeated forced ignition attempts can stress the PCB and burner components.
- Do not attempt to inspect or touch gas components Gas Safe engineer
The gas valve, burner, flame sensor, and associated wiring must only be accessed by a Gas Safe registered engineer. These are not homeowner-serviceable items under any circumstances.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
An engineer will carry out a structured fault-finding process: verifying gas supply pressure at the appliance, testing the gas valve electrically, inspecting and cleaning or replacing the flame sensor, and checking wiring continuity. They will advise on the most cost-effective repair route before proceeding.
- Gas valve replacement if confirmed faulty Gas Safe engineer
If the gas valve assembly is the confirmed cause, the engineer will source and fit a compatible replacement valve, verify gas tightness, and carry out a full combustion analysis before signing off the repair.
- PCB inspection and replacement if required Gas Safe engineer
Should all gas-side components test as serviceable, the engineer will assess the PCB for signs of damage or component failure. PCB replacement is a last-resort step and will be recommended only when diagnostics point clearly in that direction.
Parts you may need
- Flame sensor / rectification electrode · from £25
- Gas valve assembly · from £150
- PCB (printed circuit board) · from £220
- Ignition lead and electrode set · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F88 fault dangerous?
Because F88 involves a combustion or gas valve problem, the boiler locks itself out as a precaution — which is actually the safe outcome. You should not attempt to repeatedly override the lockout with resets, as this can force ignition in conditions where the boiler has determined it is not safe to run. Leave the boiler off and call a Gas Safe engineer promptly.
Can I fix the F88 fault myself?
Very little of the F88 diagnosis and repair is safe for a homeowner to carry out. You can check that your gas supply is on and attempt a single reset — that's the limit. Everything beyond that, including inspecting the flame sensor, testing the gas valve, or touching any wiring inside the boiler casing, must be done by a Gas Safe registered engineer.
How much does it cost to fix an F88 on a Glow-Worm boiler?
Most people with an F88 fault pay somewhere between £120 and £380 all-in, depending on which component needs attention. A flame sensor clean or replacement sits toward the lower end; a gas valve replacement typically costs in the region of £250–£300 including labour. PCB replacement can push costs higher — sometimes £400 or more — but this is less commonly the root cause. Ask the engineer for a written estimate before authorising any parts.
Why does my Glow-Worm keep showing F88 after a reset?
If F88 returns quickly after a reset, it means the underlying fault is still present and the boiler is doing its job by locking out again. Common repeat offenders include a flame sensor coated in combustion deposits (which prevents it reading the flame correctly) or a gas valve that is beginning to fail. Each time you force a reset without fixing the root cause you risk additional wear. Book an engineer rather than continuing to reset.