Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Boiler ON/OFF Switch ON
CH Temperature Selector OFF
DHW Temperature Selector OFF
Remove lower front facia panel
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Voltage across terminal marked 1 (terminal furthest left) and terminal marked 4 (third terminal from the left) on the connector marked ST13 on the main PCB.