Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove & disconnect internal programmer/receiver assembly from the boiler (several types can be fitted)
CH demand from any external controls fitted, i.e. room stat etc
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With internal programmer/receiver disconnected, does the boiler light for a heating demand ?