Boiler
E12
The E12 error code on a Glow-worm boiler indicates a flame detection fault — the boiler is either failing to ignite or losing its flame shortly after lighting. This is a lockout fault, meaning the boiler has shut itself down for safety and will not restart without intervention.
What does this error mean?
Glow-worm boilers use an ionisation sensor (flame sensor) to confirm that a flame has been established after the gas valve opens. When the E12 code appears, the boiler’s control board has attempted ignition and cannot confirm a stable flame is present. The boiler locks out to prevent unburned gas from accumulating, which is a critical safety function.
Est. repair cost
£100–£200 for a Gas Safe engineer call-out plus parts (flame sensor, electrodes). Boiler replacement: £1,800–£2,800 installed.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check your gas supply: Before anything else, confirm other gas appliances in your home (hob, gas fire) are working. If they are not, contact your gas supplier — the fault is not with your boiler.
Step 2 — Perform a boiler reset: Locate the reset button on your Glow-worm boiler (usually a flame symbol or marked ‘Reset’). Hold it for 3–5 seconds and release. Allow the boiler 2–3 minutes to attempt ignition.
Step 3 — Try the reset a second time: If the first reset fails and the E12 returns, attempt one more reset. If the fault clears and the boiler runs normally, monitor it closely over the next 24 hours.
Step 4 — Do not attempt a third consecutive reset: Repeated failed resets can cause unburned gas to accumulate. If the E12 returns after two resets, stop and call a Gas Safe registered engineer.
Step 5 — Do not attempt any further DIY: Components such as the flame sensor, ignition electrodes, gas valve, and PCB must only be inspected, cleaned, or replaced by a Gas Safe registered engineer. This is a legal requirement in the UK — unqualified work on gas appliances is illegal and dangerous.
📞 When to call a professional
Call a Gas Safe registered engineer if the E12 error returns after two reset attempts, or if the boiler repeatedly locks out over a short period. Tell the engineer you have a Glow-worm boiler showing error code E12 and that it is failing to establish a flame. If you smell gas at any point, do not reset the boiler — leave the property, open windows, and call the National Gas Emergency Service on 0800 111 999 immediately.
⚖️ Repair or replace?
A Gas Safe engineer call-out typically costs £80–£120, with parts such as a flame sensor (£20–£50) or ignition electrode (£15–£40) on top — most E12 repairs come in at £100–£200 total. If your Glow-worm boiler is over 10–12 years old and has suffered repeated ignition faults, replacement with a modern A-rated boiler (typically £1,800–£2,800 fully installed) is likely more cost-effective than repeated repairs. For a boiler under 8 years old in otherwise good condition, repair is the right first step.