Boiler
E3
The E3 error code on a Glow-worm boiler indicates an overheating fault — specifically that the boiler’s temperature has exceeded its safe operating limit. This is a safety lockout and should not be ignored, as running an overheating boiler can cause serious damage or create a hazard.
What does this error mean?
The E3 code means your Glow-worm boiler has triggered its high-limit thermostat or overheat sensor, detecting that water temperature inside the heat exchanger has climbed dangerously high. The boiler locks itself out automatically to prevent damage to internal components or, in the worst case, a pressure or scalding incident. It will not restart until the underlying cause is identified and resolved.
Est. repair cost
£150–£450 for professional repair (pump, sensor, or power flush); pressure top-up and bleed is DIY at no cost
🔍 Likely causes
🛠️ Step-by-step fix
Check the boiler pressure gauge — it should read between 1 and 1.5 bar when cold. If it is below 1 bar, top up via the filling loop until it reaches the correct level, then attempt a reset.
Reset the boiler — press and hold the reset button (usually marked with a flame or ‘R’ symbol) for 3–5 seconds. If the boiler fires up and runs normally, monitor it closely for 30 minutes to see if the E3 returns.
Check all radiator and system valves — walk around your home and confirm every radiator valve and any isolating valves on pipework are fully open. A partially closed valve can starve the boiler of flow.
Bleed your radiators — turn off the boiler, use a radiator key to release any trapped air from each radiator starting upstairs, then repressurise and try again. Airlocks are a common cause of E3 on Glow-worm units.
If the E3 error returns after these checks, do not attempt further DIY. This is a gas appliance and any work beyond basic resets, pressure top-ups, and bleeding is legally required to be carried out by a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the E3 error returns after a reset, or if the boiler feels excessively hot to the touch, is making unusual noises, or is showing signs of a leak. Tell the engineer: ‘My Glow-worm boiler is showing an E3 overheat lockout fault’ — this will help them bring the right parts, likely an NTC sensor, pump, or inhibitor flush kit. Do not keep attempting resets on a repeatedly locking-out boiler.
⚖️ Repair or replace?
Most E3 repairs — pump replacement, system power flush, or sensor swap — cost between £150 and £450 depending on the fault. If your Glow-worm boiler is over 10–12 years old and has required multiple repairs, replacement with a modern A-rated combi boiler (typically £1,800–£2,800 fully installed in the UK) is almost always the more cost-effective long-term decision. For boilers under 8 years old in otherwise good condition, repair is strongly recommended.