Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Connect the air pressure switch charge kit to +ve connection on the air pressure switch
(35 CDi II appliances may have APS fitted inside combustion
chamber next to fan - confirm gas is OFF before continuing)

WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Note the operating pressure indicated on the pressure switch.
Allow fan to run.
Once fan begins to run increase the pressure via
the syringe to that indicated on the pressure switch.
Sparking should then be observed through the sight glass.
Note:
If sparking is not observed it is permissible to increase the pressure by 50%.
100 Pascals = 1mbar / 50 Pascals = 0.5mbar