Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Selector switch to DHW position
CH temperature selector to minimum
DHW Preheat selector to OFF position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run. After approximately five seconds measure voltage across connection 3 (blue lead) and connection 5 (brown lead) on connector A1 on the PCB.
Voltage will only be present for approximately 10 seconds during ignition sequence.
If boiler goes to lockout it will require resetting.