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:

Allow burner to ignite and establish.
Measure the DC voltage between the white wires (30 & 31) at the right hand plug on the PCB.
Approximately 20V DC should be measured.