Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to DC Volts 
CH Demand
Main On/Off Control in the On position
CH Temperature Control to Maximum
CH Temperature Control to Maximum
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
If a fault code is displayed press boiler 'Reset' button.
Measure DC Voltage across the backmost black wire on the small orange plug (X3) to a suitable earth point.
Voltage should be approximately 4.0V to 7.0V DC.
Measure DC Voltage across the backmost black wire on the small orange plug (X3) to a suitable earth point.
Voltage should be approximately 4.0V to 7.0V DC.
Isolate as per TOPs P4
and refit as necessary