The fan motors rpm speeds are monitored through the remote. Two channels: MM#10 and #20. In previous generations MM#49 is also used.
MM#10 = Calculate Fan Speed
MM#20 = Actual Fan Speed
MM#49 = Fan Speed Correction associated to installation conditions and dipswitch settings.
A good indication that your fan motor needs to be serviced can be confirmed by the discoloration of a blue flame in the burner chamber. The majority flickers of yellow, orange, and red hues indicates less than ideal combustion conditions associated to installation conditions like: high elevation, high gas pressure supply, and blockages associated with too long vents or accumulation of exhaust gases
The Fan Motor can also be removed to provide a more extensive service cleaning associated to poor combustion air quality, high elevation, longer vent runs, and general neglect.