Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove the fan assembly

Observation:

Visually inspect the condition of the air pressure switch tubes and pipes, they should be free from any splits / damage, corrosion or any restrictions or blockages.
Also check the small black test point cap is fitted and sealed.
Are the air pressure switch tubes and pipes satisfactory?
Note:
It is not possible to gain full access to the lower APS pipe as it cannot be removed and is located behind the flue hood / fan mounting assembly, however it can be visually inspected from the top of the appliance using a mirror and torch and it is possible to confirm it is not restricted or blocked by pushing a piece of wire through it. It should be noted there is not normally an issue with this pipe as it is situated in the incoming air.