Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
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 the Grey wire (connection 3) and the Brown wire (connection 12) at PCB connector PL3.