Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Boiler On/Off Switch ON
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the Voltage across connection 1 (Brown wire) and connection 3 (Blue wire) on connector PL6 on the PCB.