Set Appliance State:
Gas Off
Set Multimeter to AC Voltage 
Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use insulation mat as necessary)
Gain access to PCB (use insulation mat as necessary)
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Measure AC voltage across connection 3 (Yellow wire) and connection 4 (Orange wire) on connector CN7 on the Main Control PCB. This voltage should be above 24V AC.
Isolate as per TOPs P4
and refit as necessary