Glow-worm F77 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F77 fault code mean?
The F77 fault code on a Glow-Worm boiler signals a gas valve problem detected by the boiler's control board. Specifically, the boiler has sensed gas combustion occurring at the burner when no heat demand has been issued — in other words, gas appears to be flowing and burning without the boiler being told to fire. This is a serious safety condition, so the boiler immediately locks out and refuses to restart until the fault has been properly diagnosed and resolved by a qualified professional. Because gas is involved, this code must always be treated as a priority fault: do not repeatedly attempt to reset the boiler, and if you smell gas at any point, turn off your gas supply at the meter and call the National Gas Emergency line on 0800 111 999 straight away.
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
- Gas valve stuck open or failing to close Common
The most likely cause of F77 is a gas valve that is not sealing properly when the boiler is at rest. If the valve fails to close fully, gas can continue to reach the burner even when no heat demand is present. The boiler's control board detects this unintended combustion and triggers the safety lockout. Mechanical wear, internal debris, or a faulty solenoid within the valve can all cause this behaviour.
- Faulty gas valve solenoid or internal component Common
The gas valve contains solenoid coils that open and close the valve on command. If a solenoid coil has failed or is producing an incorrect electrical response, the valve may behave unpredictably — staying open when it should close, or providing false feedback to the PCB. This often requires full valve replacement rather than a component-level repair.
- Wiring fault between PCB and gas valve Sometimes
Damaged, corroded, or loose wiring in the loom connecting the printed circuit board to the gas valve can cause incorrect signals to pass between the two components. The PCB may believe the valve is open when it is not, or vice versa, triggering the F77 fault. A Gas Safe engineer will check connector integrity and continuity during diagnosis.
- PCB sending incorrect control signals Sometimes
The printed circuit board governs when and how the gas valve operates. If the PCB develops a fault — whether due to a power surge, component failure, or moisture ingress — it may send erroneous signals to the valve, causing the boiler to detect a gas flow discrepancy and lock out with F77. This is a less common cause but should be considered if the valve and wiring check out as serviceable.
How to fix it
- Switch the boiler off at the wall or programmer immediately DIY safe
As soon as F77 appears, power the boiler down. Do not attempt to reset and refire it. Because the fault indicates possible uncontrolled gas flow, leaving the boiler running or cycling resets is not safe practice.
- Check for a gas smell and act accordingly DIY safe
Go near the boiler and check whether you can detect a smell of gas. If you can, do not touch any electrical switches. Turn off the gas supply at the meter, open windows and doors to ventilate, leave the property, and call the National Gas Emergency line on 0800 111 999. Do not return until emergency services confirm it is safe.
- Confirm your gas supply is functioning normally DIY safe
If there is no smell of gas, check whether other gas appliances in your home — such as a gas hob or fire — are working as expected. If nothing is getting gas, the problem may be with your supply rather than the boiler itself. Contact your gas provider if you suspect a supply interruption.
- Do not attempt to reset the boiler more than once DIY safe
A single reset attempt is reasonable to check whether the fault clears, but F77 is a safety lockout related to gas valve behaviour. Repeatedly resetting the boiler without understanding the underlying cause is unsafe. If the code returns after one reset, leave the boiler off and move on to calling an engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
All further work — including testing the gas valve electrically, inspecting solenoid coils, checking wiring continuity, and assessing the PCB — must be carried out by a Gas Safe registered engineer. They will use specialist diagnostic equipment to identify whether the valve itself, the wiring harness, or the PCB is the root cause, and will advise on the appropriate repair or replacement. Ask the engineer to carry out a general boiler health check at the same time, particularly if your boiler is several years old.
Parts you may need
- Gas valve assembly · from £220
- Gas valve solenoid coil · from £55
- Wiring loom / harness (gas valve section) · from £45
- Printed circuit board (PCB) · from £280
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 £250–£500, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F77 fault dangerous?
It can be. The F77 indicates the boiler has detected gas burning without a heat demand being made — which points to the gas valve not controlling flow correctly. This is a safety-critical condition, which is why the boiler locks out rather than continuing to operate. You should not try to reset and run the boiler repeatedly. If you smell gas at any point, treat it as a gas emergency and call 0800 111 999. In the absence of a gas smell, the immediate risk is lower, but the boiler must still be inspected by a Gas Safe engineer before it is used again.
Can I reset my Glow-Worm boiler to clear the F77 code?
One reset attempt is reasonable, but if the F77 returns — which it almost certainly will without a repair being carried out — leave the boiler off. The fault is a protective lockout designed to prevent unsafe operation, and bypassing it with repeated resets does not fix the underlying problem. An engineer must identify and rectify the root cause before the boiler will operate reliably and safely again.
How much does it cost to fix a Glow-Worm F77 fault in the UK?
For most homeowners, the repair involves replacing the gas valve, which typically costs between £250 and £500 including parts and a Gas Safe engineer's labour. The gas valve component itself usually sits in the £140–£300 range depending on the boiler model, with labour adding £100–£200 on top. If the PCB also turns out to be faulty, costs will be higher — PCB replacements can add £250–£400 to the bill. If your Glow-Worm boiler is more than 10–12 years old and repairs are becoming frequent, it may be worth discussing a full boiler replacement with your engineer, which typically costs £1,500–£2,800 installed.
What is the difference between the Glow-Worm F26 and F77 fault codes?
Both F26 and F77 relate to gas valve faults on Glow-Worm boilers, and they share a similar root cause — the control board detecting a problem with how the gas valve is behaving. F77 specifically indicates that combustion has been detected when the boiler was not commanded to fire, suggesting gas is flowing through a valve that should be closed. F26 can indicate a related valve control error. In practice, both codes require the same response: switch the boiler off, do not repeatedly reset it, and arrange for a Gas Safe engineer to inspect and repair the gas valve circuit.