On my Carrier furnace (model 58G5100-2) the igniter starts the pilot, and most times the pilot starts the burners after a minute or two and then the blower motor starts.

Other times, pilot lights the burners which burn for a minute or two, stop and then start again... and after a few minutes the blower motor starts and runs (the burners are on).

Then sometimes the burners shut down after a minute or two and start up again. This goes on and off... with no specific pattern, sometimes not happening at all.

I have tried replacing the "limit switch", but that did not correct the problem... any suggestions?

I understand fairly well how furnaces operate, but do not have enough experience to know the details of yours from the model number. You have 3 controls that govern the gas burner, the thermostat, the limit switch, and a thermocouple or flame detector. There may be 2 limit switches, one to turn on the blower after the air in the furnace is warm, and another that shuts the gas off if the furnace over heats because the blower isn't running. The third control shuts off the gas if it isn't burning. It sounds like one of the last 2 controls is flaky, letting the gas continue to burn sometimes, and not other times. Thermocouples frequently are a problem, but cheap and easy to replace. They are a 1/8'' tubing going from a flare fitting to a 1/4'' X 2'' cylinder that sticks into the pilot flame. I think newer furnaces use a different control, but can't describe one. It will be right where the flame hits it. Before replacing anything, clean and tighten all the connections. Clean the outside of the thermocouple if it has carbon or other crud on it.

These intermittent things are a pain to trouble shoot. You may have to have a meter connected to the circuit when it fails to identify the bad component. The problem could also be on a control circuit board.

Mr. Labman,

I was having the same problem and I clean the thermocouple and work just perfect thanks for your help.