Boiler
E4
The E4 error code on a Glow-worm boiler indicates a fault with the ignition — specifically, the boiler is failing to detect a flame after attempting to light. This is a lockout fault, meaning your boiler has shut itself down for safety and will not restart without intervention.
What does this error mean?
When your Glow-worm boiler displays E4, it has tried to ignite the burner and failed to establish or confirm a stable flame signal. The boiler’s control board monitors ignition via an ionisation probe; if no flame is detected within the allotted ignition attempts, it locks out and displays E4 to protect you from unburned gas building up. This is a safety lockout — the boiler will not fire again until the fault is resolved and the unit is reset.
Est. repair cost
£80–£200 for a Gas Safe engineer repair (parts + labour); new boiler replacement £1,800–£3,000 installed
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, cooker) are working. If not, call your gas supplier — there may be a network outage or your meter may need topping up.
Inspect the condensate pipe: in cold weather, locate the white plastic pipe (usually exits through an external wall). If it’s frozen, thaw it carefully with warm — not boiling — water and a warm cloth. Glow-worm provides guidance on this in the user manual.
Reset the boiler: press and hold the reset button (typically marked with a flame or ‘R’ symbol) for 3–5 seconds, then release. Allow the boiler a full ignition cycle (up to 60 seconds) to attempt restart.
Repeat the reset once more if the first attempt fails — a single transient fault can sometimes cause a lockout that clears on second reset.
If the boiler locks out again after two reset attempts, do not keep resetting. Repeated failed ignitions can allow unburned gas to accumulate. Stop here and call a Gas Safe registered engineer.
Do NOT attempt to access the boiler casing, touch electrodes, probes, or gas components yourself. This is illegal for unqualified individuals and extremely dangerous.
📞 When to call a professional
If the boiler locks out with E4 more than twice after resetting, or if you smell gas at any point, call a Gas Safe registered engineer immediately — and if you smell gas, also call the National Gas Emergency Service on 0800 111 999. Tell the engineer you have a Glow-worm boiler showing an E4 lockout fault and that it is not sustaining ignition; this allows them to bring the most likely replacement parts (electrodes, ionisation probe, gas valve) to the visit.
⚖️ Repair or replace?
A Gas Safe engineer call-out and repair for an E4 fault typically costs £80–£200 depending on the part required — electrodes and probes are cheap, a gas valve or PCB replacement will push costs higher. If your Glow-worm boiler is over 10–12 years old and this is not the first significant fault, repair costs can quickly approach the price of a new boiler (£1,800–£3,000 installed). For older boilers with a history of faults, replacement is usually the smarter financial decision.