Boiler will not ignite, display shows E01 shortly after a heating or hot water demand.

Safety first: Switch the boiler off at the fused spur and isolate the gas supply if you smell gas. Do not attempt gas work if you are not Gas Safe registered.

Basic checks (user level):

• Confirm other gas appliances (hob, fire) work. If not, contact your gas supplier.
• Check the boiler’s system pressure (if sealed system) is around 1.0–1.5 bar when cold. Top up via the filling loop if it is below 1.0 bar, then reset the boiler.
• Ensure the boiler’s external controls (room thermostat, programmer) are calling for heat and that the boiler is not in ‘summer’ or ‘standby’ mode.

Technical checks (engineer level):

• Inspect the ignition electrode and lead for cracks, carbon build-up or damage; clean with fine emery cloth and replace if worn.
• Check the flame sensor (ionisation probe) for contamination or damage; clean or replace as required.
• Verify correct polarity and earth continuity to the boiler PCB.
• Measure gas inlet and burner pressure and compare with EHC boiler specifications; adjust or replace the gas valve if out of range.
• Check the flue for blockage or incorrect termination that may affect combustion.

If the boiler repeatedly locks out on E01 after these checks, the ignition circuit on the main PCB or the gas valve may require replacement by a qualified engineer.