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:

Measure for DC voltage across pins 1 and 4 on connector X2 at the PCB.
This voltage will vary according the boiler system pressure.
Approx voltage should be 1.7 to 1.9 volts DC.
System pressureVoltage DC
0 bar1.2
1 bar 1.7
2 bar2.2
3 bar2.8

Note:
Colour of wires may change but test points remain the same.