My ducting in the floor was crushed. Poor airflow was an indicator but the real issue is inadequate airflow for the furnace to operate properly. Mine would cycle the burner due to overheating. The high limit switch does this and should not.
I did everything to get the duct propped open and clear. In the end the solution was to add two more 4" round ducts through the cabinet. Makes an amazing difference. Burner now runs continuous until the thermostat is happy.
I looked down the ducting with mirrors and removed the furnace itself to inspect that area. My ducting is between a double floor and I have no belly cover so no real access. Not sure you would even have access if you removed the plastic cover.
