Lennox furnace serial 6396j0178 model 80mfg3-75a-5

A cleaning probably will buy time. Video: http://www.youtube.com/watch?v=I86joWL66aE Clean with emery cloth, not sand paper.

already cleaned...

It's been cleaned twice. Yesterday, cleaned again. Worked for a couple hours and than started shutting down again.

The flame sensor is just a steel rod with a ceramic insulator. If the ceramic isn't cracked and the rod is clean than the problem is not the flame sensor. Rust on the end of the burners or a poor ground will also interfere with the control boards ability to sense flame being present.

espicially with that furnace. I have even had to run a green groundwire from one of the screws on the burner box directly to the circuit board ground to make them work better.