Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the air impulse tube from the air gas manifold
Connect a performance tester to the air gas manifold test point
(where the impulse tube was previously connected)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the fan pressure during the fan and pump purge period.
Expect approximately 2.6mb.
Is the fan pressure as expected?