Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the leads from the air pressure switch
Remove connector CN1 from the PCB

Measurement:

Using the air pressure switch testing kit, increase the pressure on the air pressure switch until short circuit can be measured across the common and normally open terminals of the switch.
With the testing kit still connected, reconnect the wires to the air pressure switch.
Measure resistance across the DHW cut-off
circuit (yellow and black wires on CN1).
Expected resistance short circuit (zero Ohms).