Boiler will not ignite, display shows E01, no flame symbol, heating and hot water not working

Safety first: Turn the boiler off at the control panel and isolate electrical power at the fused spur. If you smell gas, do not attempt any repair—ventilate the area and contact your gas supplier or a Gas Safe registered engineer immediately.

Basic user checks:

• Verify that the gas supply is on (check other gas appliances such as a hob).
• Check system pressure is around 1.0–1.5 bar when cold; top up via the filling loop if it is below 1.0 bar.
• Press the boiler reset button once and wait a full ignition cycle.
• Ensure the room thermostat and programmer are calling for heat.

Technical checks (for qualified engineers only):

• Inspect the ignition electrode and flame sensing electrode for cracks, carbon build-up, or damage; clean with fine emery cloth and check electrode gap to manufacturer spec.
• Check the ignition lead for signs of arcing or insulation breakdown; replace if suspect.
• Verify correct polarity and earth continuity to the boiler.
• Measure gas valve coil resistance and confirm correct voltage during ignition; if the valve does not open, test and replace the gas valve assembly.
• Confirm adequate inlet gas pressure and burner pressure according to Potterton data plate; adjust or rectify supply issues as required.

If E01 persists after basic checks, a Gas Safe registered engineer should diagnose and replace the faulty ignition or gas components.