Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset boiler if required.
Measure the DC voltage across pins 33 & 35 on connector CN1.
Expect approximately 10 Vdc or greater.