Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove yellow wire from the condense trap and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

User control set to ON position.
Run the appliance again, does the burner establish correctly now?
Note:
Ensure the yellow wire cannot touch any of the appliance components or chassis.