Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Remove the white wire from the condensate trap assembly and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow ran to run and burner to ignite.
Does the burner now establish?
(i.e. flame remains alight and sparking ceases)