Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4

Observation:

Inspect the condition of the air pressure tubes running from the fan venturis to the APS.
They should be clear, unrestricted and free from any damage (splits, cracks or kinks).
Are the air pressure tubes in good condition?
Also check the orientation of the tubes, the top tube should go to the negative pressure connection on aps and the bottom tube should go to the positive pressure connection on the aps.
Note:
80 & 90 versions have one tube running from a venturi in the rear of the flue collector to the APS as indicated by the blue arrow which should go to the negative connection of the APS.