Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Selector Switch in Summer (I) position
Remove the two black wires from the air pressure switch and make safe.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Press and hold in the ignition button.
Measure the resistance across the
COMMON and N.O. terminals