Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect blue and yellow leads from the system pressure switch
Disconnect blue and yellow leads from the system pressure switch
Measurement:
Measure resistance across the exposed terminals of the system pressure switch.
Expected reading closed circuit.
Expected reading closed circuit.
Isolate as per TOPs P4
and refit as necessary