Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Use insulation mat as necessary
Loosen connector for fan slightly to expose test pins
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Using the test pins check for mains voltage between the black and red wires on ST1.
Note:
Boiler sends mains voltage between black and red for
DHW and black and white for CH.