Boiler locks out with EA on display, burner does not ignite, you may hear clicking from the ignition but no flame established.

Safety first: Switch the boiler off at the fused spur and isolate the gas supply at the service valve before removing any covers. Only Gas Safe registered engineers should work on gas-carrying components, combustion chamber, or flue.

Basic user checks:

• Confirm gas supply is available: check other gas appliances (hob, fire) are working. If not, contact your gas supplier.
• Check the boiler pressure gauge is around 1.0–1.5 bar when cold. If low, top up via the filling loop according to the user manual, then reset the boiler.
• Inspect the flue terminal outside for obvious blockage (ice, debris, bird guard obstruction). Do not dismantle the flue.
• Press the reset button once and observe if the boiler lights.

Technical checks (engineer only):

• Remove the combustion cover and inspect the ignition and ionisation electrodes for cracks, heavy oxidation, or incorrect gap. Clean lightly with fine emery and refit, or replace as a set if worn.
• Check the ignition lead for damage, tracking marks, or loose connections at the PCB and electrode.
• Measure ionisation current during operation; compare with Worcester Bosch specifications. Replace the flame sensor/ionisation electrode if readings are low.
• Verify correct operation of the gas valve: check supply pressure, inlet filter, and that the valve opens on demand. Adjust or replace the gas valve as per manufacturer’s combustion settings using a flue gas analyser.
• Inspect the flue system for correct assembly, slope, and integrity; rectify any condensate pooling or restrictions.