I replaced the thermostat,fan limit control switch, and still when a/c is turned on the fan shuts on and off every 30-45 second intervals, sometimes it will run contiuously, sometimes it won't. And when I say the fan limit control switch It's the part that has pointers on it for dgree on start and shut down. Any help would be greatly appreciated thanks. It`s a Lennox furnace, I had a a/c contractor come in and he did the replace meant parts. Problom not solved, contractor will not come back with out billing me another service call. When I explain the problem to other contactors they won't touch it... could it be the fan motor? There is no fan relay switch on this system. Why I do not know , but fan setting is on high speed,(needed for a/c).

Yes it could be the motor, can be checked with a volt meter on the wires to the motor to verify constant voltage to the motor when the motor shuts down, could be a run capacitor on the indoor motor (much cheaper then the motor) Could also be the fan relay which is a different componet then the fan- limit switch. The fan-limit switch works in conjunction with the heat to bring on the fan during the heating mode and shut off the gas valve (limit) should the furnace overheat. Mike

What is the model number of the furnace?