Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Gain access to PCB (use Insulation Mat as required)
NO Demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Test 1:
Measure the DC voltage between Pin 12 (Green) on X25 and a suitable earth.
Expect approximately 1.4 Vdc
Test 2:
Apply a CH demand, allow the fan to run and measure the DC voltage as above again.
Expect approximately 2.5 Vdc or greater.