Gas furnace, 80% effiency. Not sure of the name brand. When furnace is running for a little while, the burners will kick off while the blower motor keeps running. After a short time the burners kick back on. This cycle repeats itself until the temperature at the t-stat is reached. Is it possible for fan limit switch to be bad?

Yes it is possible the fan and limit is bad or the filter could be plugged up are the return air could be blocked or to many registers could be closed off trying to save energy.

The filter is brand new. How can I check if there is enough return air. I will check if there are any vents that are closed. Thank you for you help.

how can i check if there is enough return air

If the furnace worked OK before with the return you have now it should be OK now.

It was. It just started doing this on Friday. I will replace the limt switch and go from there. Thanks again.

Post back with the results.

I changed the fan limit, but to no avail. I should have been smarter, because after I changed it I then realized that the heat exchanger temperature is not even getting very hot before the burners turn off. The flame turns on only for a short time and then turns off. I had to turn the blower motor start temp on the fan limit way down just to get the fan to blow what warm air there is out of the furnace. I'm now leaning to the flame sensor, but if is not the problem, I don't know what to look for after that.

Cleaning the flame sensor is a good idea. You are running into the typical problems associated with a intermitent situations. The flame sensor if not working properly can cause the unit to cut off.

BUT try this first. In the furnace there is a strip for landing the thermostat wires. Disconnect R and W and put a jumper across R and W. This will eliminate the thermostat and thermostat wiring from the circuit. It could be a thermostat problem that is causing the furnace to short cycle. Use the furnace switch for the power to turn the furnace off and on for testing purposes and naturally turn the switch (power) off while hooking up the jumper. Post back with results.

Its not a problem with the flame sensor. It appears to be working properly. Before the burners turn off, the relay on the gas valve clicks a few times and then they go out. How do I check to see if the relay on the gas valve is bad, or is it trial and error. I guess the only other possible thing would be the board. Is there a way to check that>

I will need the exact brand of furnace
I also need the exact model number

Did you do the R and W test above to eliminate the thermostat and wiring.

BUT try this first. In the furnace there is a strip for landing the thermostat wires. Disconnect R and W and put a jumper across R and W. This will eliminate the thermostat and thermostat wiring from the circuit. It could be a thermostat problem that is causing the furnace to short cycle. Use the furnace switch for the power to turn the furnace off and on for testing purposes and naturally turn the switch (power) off while hooking up the jumper.

I did jump the t-stat terminals together, and it did the same thing. The furnace is a trane xl 80. Model # is tud120b96oao Tomorrow I want to check the gas valve out. There are 3 leads that go into the terminals on the valve. I'm just not sure what ones to check for the voltage.

The last place I would look for trouble is the gas valve.

Changing the board today. I will let you know what happens. The gas valve was getting correct voltagewhen flame was there and showed no voltage when the flames were not present. The flame sensor is clean and working fine. So hopefully this will correct it. If not, I have a big sledge hammer that I intend to take full advantage of.