Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove & disconnect internal programmer/receiver assembly from the boiler
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Ensure preheat is On " Preheat" displayed, if not press ECO button (approximately 1 second) display should display Preheat.
With the internal programmer/receiver disconnected,
does the boiler light for a preheat demand ?