Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to the outlet of the fan assembly

Observation:

If the horizontal or vertical flue lengths are less than 1 metre, the diaphragm on the fan outlet must not be present / removed.
The diaphragm can be reached by removing the fan
assembly, see figures 6a, 6b &6c.
Is the diaphragm installation correct for the flue length of appliance?
Note:
Depending on boiler output the diaphragm is a different size , if you suspect the wrong sized diaphragm has been fitted, then the fan assembly will need to be removed to accurately check.
Heatline C24 : 41mm
Heatline C28 : 44mm