Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
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 1 (Blue) on connector X25 and a suitable earth.
Expect approximately 24 Vdc.
Test 2: Measure the DC voltage between pin 2 (Red) on connector X25 and a suitable earth.
Expect approximately 24 Vdc.
Test 3: Apply a CH demand.
When the boiler reaches the sparking stage of the ignition cycle measure the DC voltage between pin 1 (Blue) on connector X25 and a suitable earth.
Expect approximately 0 Vdc