Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Main On/Off Control in the On position
CH Temperature Control to Maximum
Remove Yellow plug X12 from Motherboard and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

If a fault code is displayed press boiler 'Reset' button.
Select Diagnostic Mode on Multifunction Display by pressing both buttons together.
Push left hand button to step through diagnostic codes until screen shows "d.10" and then press the right hand button once. The display should now show "1".
Reset Display