Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
DHW temp selector to the high setting
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Boiler On/Off switch On.
Reset lockout if necessary.
Allow fan to run and ignition sequence to commence.
Can the burner be heard to ignite and establish?
Note:
On some versions the flame can be seen via the viewing
window at the top of the heat exchanger.