Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage between blue (7) on connector X401 and earth as power reinstated.
Expect approximately 17Vdc for the first 5 seconds after the boiler is powered on.
Is the voltage measured as expected?
Note:
If fused spur is not located near appliance selector switch can be used to turn appliance on and off.