Set Appliance State:

Mains Power Off
CH Demand
Boiler on/off switch ON
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset lockout as necessary.
Allow the fan to run and ignition sequence to commence.
Does the burner ignite?
Burner can be seen through sight glass.
Note:
On later models the sight glass may not be there, if it’s not then you will have to listen for the burner igniting.