Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Disconnect rectification lead from condensate sensor, place it so it cannot short against any metal
(sensor may be fitted on top or at the side - if fitted to top you
may need to unscrew condensate to give enough room to remove)

WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance if required (rotate selector switch to
'O' position, then rotate back to DHW/CH position).
Allow self-check to complete and ignition sequence to commence.
Does the burner now establish?