Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Main On/Off Control in the On position
Heating Control to Heating & Hot Water
CH Temperature Control to Maximum
Remove Yellow connector X12 from Motherboard and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

If a fault code is displayed press boiler 'Reset' button
.
Measure DC Voltage on across X12 on Motherboard.
This should lie between 20.0V and 30.0V DC.