Furnace is installed in a trailer without central air.(heat only) Trailer had been unoccupied with heat turned down very low. Went to work @ trailer and heat worked fine. When I left, again turned heat down. Went back a week later and heat would not come on. T-stat called for heat, ignigtor lit, combustion fan runs and burner lights, blower fan comes on for short time and burner goes out. Fan continues to run for short time and shuts off. This cycle will repeat several times. Replaced t-atat and filters are clean. Same problem. Any idea what might be wrong?

Sounds like the flame senser may be dirty. Try cleaning it with steel wool, sandcloth, or fine sandpaper. Careful not to touch or bump the glow plug as they are very fragile and break easily. The flame senser will be located on the opposite side of the glow plug on the burner. Just the metal rod that sticks in front of the flame.

Thank you! By the glow plug I assume you are referring to the ignigter? Is there any cautions to removing the burner assembly to acess the flame sensor?

Yep, by glow plug, I mean the hot surface igniter. As for removing the burner assemly, make sure gas is off, all wires are disconnected(igniter, flame senser, and gas valve. Should be a total of five wires. Then, when actually taking out the burner, just be very careful not to let the igniter hit anything. Other than that, just remember the gas valve orientation and placement of wires when reinstalling. Most likely you have a flex gas supply. If this is the case, then you don't need to disconnect the gas line. However, if it is hard piped, you will need to break the union to remove the gas valve(turn gas off either way). Good luck and let me know if this cures the problem.

Thanks again! Your direction was right on and everything is working fine. I appreciate your valuable help.

Glad to hear you got it fixed. Thank you for the update.