Bryant furnace "Plus 90i" will run, flash red light a couple of times, shut down, then gear up and run again. It can do this 4-6 times over the course of several hours while heating the house up to 68 or 70 degrees from an overnight temp of 61 degrees. The service techs have checked the programmable thermostat and the furnace. The last tech declared there was nothing wrong with the furnace and the problem is external. Our outside temperatures reach -30 C and we have 2 inch intake piping. However, I cannot see or feel any icing at the external end of this pipe. This pipe is also heated with a wire. Furthermore, this problem was occurring when the outside temperatures were still above freezing.

Bypass the tstat,, take loose the red and white wires where they connect to the furnace and make a jumper wire to connect the post that had the tstat wire connected,, if the furnace runs constant without stopping or turning off,you will know the problem is in the tstat or the wiring between the furnace and the tstat