Set Appliance State:

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

Observation:

With no C/H or DHW demand present, is the Fan turning slowly?
NOTE: Selector Switch must not be in the 'O' (OFF position). It must be in the C/H or DHW position.
ALSO: Do not take noise or vibration as an indication of operation.