Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Locate system filing loop
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Test 1:
Measure for DC voltage between pin 14 (Green) on X25 and the
earth point on the boiler chassis - make a note of the voltage measured.
Test 2:
Slightly Increase the system pressure and repeat Test 1.
Has the voltage increased after the increase in system pressure ?
Note:
Each pin on the PCB connector is numbered.