Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
Main On/Off Control in the On position
Disconnect connector X10
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.
Test 1:
Measure between 8 and 12 V AC across connections 1 and 2 of plug X10.
Test 2:
Measure between 18 and 22 V AC across connections 3 and 4 of plug X10.
Note.
All voltages are made on the end of the plug and not on the Motherboard.