Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
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 and pin 2 on
connector X9 - make a note of the voltage measured.
Test 2:
Slightly Increase the system pressure then repeat Test 1.
Has the voltage increased after the increase in system pressure ?