Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure appliance is cool - below 30°C
(This can be achieved by running the DHW with the gas off)
Attach a temperature clamp to the flow pipe as indicated by the red arrow
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Turn the selector switch to the preheat setting.
The appliance should ignite and run for preheat, heat up to
approximately 50°C then the burner should extinguish.
Does the preheat operate satisfactorily?