Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Allow boiler self-check & purge cycle to commence
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

During the boiler self-check & purge cycle;
Test 1.
Measure for DC voltage across pin 3 CN8 on PCB & a suitable earth connection.
 
Expected voltage approximately 28 Vdc.
Test 2.
Measure for DC voltage across pin 2 CN8 on PCB & a suitable earth connection.
 
Expected voltage should be seen to fluctuate between 0 - 2.0 Vdc approximately.