My furnace will turn on and blow cold air at times. When I notice it I can turn it off on the thermostat and then turn it back on check the furnace and it lights up and blows hot air. I have also just pulled the panel off the furnace to trip the switch on it and then put the panel back on and the heat starts up again. It seems to be an intermittent problem on self start up.

Any ideas?

Clean the flame sensor. Start with that.

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Originally Posted by MarkwithaK

Clean the flame sensor. Start with that.

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OK I just cleaned it afterwards, I put the panel back on and sure enough no flame. I took the panel back off then popped it back on and it lit up and is blowing now. Guess I'll find out if it stays that way.

So it got to the desired temp and the heat cut off, but the blower continued to run. I turned the thermostat to the off position and the blower kept going , finally I had to pull the panel off. It turned off I put the panel back on and then flipped the switch on the thermostat to the heat position. I'm now waiting ofr it to get cold enough to kick back on so I can see if it does it again. Is it possible that the thermostat is the problem? How can I check?

OK so I went out a bought a thermostat just for the heck of it. And just as I thought it did nothing for the situation.

Basically the gas starts up and stays on until the house is warmed up to my desired temp. Then the heat goes off and the fan stays on. I cannot turn it off from the thermostat. I have a Tran X330935P01 furnace. I believe I do not have a fan control switch , I have a Control board. I'm starting to lean towards that piece as my problem , however If someone could tell me how to trouble shoot it that would be great. I did short the white to red and got the gas to start and I did the green to red and got the fan to turn on. Neither showed any problems. I also took off the flame sensor and lightly sanded it down with very fine graded sandpaper. It was very clean to begin with. So I do not know if there is something else I should be looking at or not. Just looking for some help here if anyone has another lead to look at.

So this morning I woke up set the desired temp on the themostat, turned to power on to the unit and the unit kicked on and heated the house. Again the house warmed up to the temp desired and the gas cut off , however the fan still stayed on. So I again cut the power off to the unit. After 30 min or so, I went to turn on the heater again, I turn off the thermostat to the off position went to turn the power to the unit back on , and the fan just kicked on and started on. The gas would not ignite , I figure because the fan is on and the control board blinked 5 times which according to the board there is no gas turning on. I believe the blinking has to do with the fan turning as I am lead to believe it will not ignite if the fan is turning. So I turned off the furnace took off the leads from the thermostat and then powered it back on, the fans immediately came back on again.

So now the fans run constantly and the gas will not turn on.

Any clues?

OK so before I go out and buy a new board, I have been doing some research and I found a thermal limit switch it looks like this:
http://www.myhvacparts.com/subsite3/...20SWT01280.jpg

Is there a way to test this ? Can I bridge the two cables running to it? Anyone?

I ended up trouble shooting it to the furnace control board.

Thanks everyone , I must say this is one lively forum. :)