I have an 18 yr old lennox gas furnace that has worked fine until this winter. If I set the thermostat to heat, the fan to auto, and set the dail up over the room temp, the gas burner lights for about 2-3 minutes... then turns off. If I flip the fan from "auto" to "on", it will fire up and heat until it reaches set temp. During this heat-up, if I flip the fan to "auto"... the fan goes off but the burners stay on... I took the honeywell bi-metallic switch out and cleaned it... still no go. I have a feeling the problem may be in this limit switch... but am guessing. Can one clean the electrical contacts in these switches? Thanks in advance for any input! Don bunker

Your fan control switch is defective. Sometimes this is called a combination fan and limit control.