Set Appliance State:

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

Measurement:

Allow fan to run.
Measure voltage across connection 7 (live, brown wire) and connection 11 (neutral, blue wire) on ignition PCB.
Note:
Connections are numbered from the top (1) to the bottom (12).