Set Appliance State:

Mains Power Off
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
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 between red wire of Programmer's connecting ribbon cable and earth.
Voltage measured should lie between 18V and 22V DC.