Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Connect the air pressure switch charge kit to
+ve connection on the air pressure switch
Note the operating pressure indicated on the pressure switch
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Ensure the performance tester reads 0mb.
Increase the pressure via the syringe to that indicated on the pressure switch.
(it is permissible to increase the pressure a further 50% if necessary)
Once pressure switch has made (audible click should be heard)
return pressure back to 0mb.
Attempt to reset boiler, does the boiler now reset?
Notes:
This fault code is known to appear when a new PCB is fitted, making the APS resets its position for the PCB and allows the board to reset.
100Pa (Pascals) = 1millibar