We have the above mentioned natural gas furnace and it is about 6 years old. For past two weeks I have been noticing that at the end of the cycle when the blower suppose to blow the air at high speed (typically this last up 40 to 50 seconds) it is lasting only 15 to 17 seconds and it dies off. I decided to monitor one day and sure enough the LED status light flashed 3 consecutive red flashes. The manuals calls for to check pressure switch.

Now the trouble shooting part - My house is 2750 sqft. So according to the municipal code, we need to have two furnaces. I have a 100,000 btu furnace for the main level and basement and a 80000 btu for the upstairs. The problem is with the 100000 btu furnace. Since the error called for the pressure switch, I switched the two pressure switches from the 80000 btu to the 100000 btu and problem still persists. What else could be the problem? I have new air filters replaced as well.

Appreciate all the help I can get.