The flame sensor and its location was not listed in the lennox manual. The sensor is under the fourth flame channel at the end of the gas pipe. It is hard to see-it should be mouted on the side, not under the channel. I found it by tracing the connection - flame sensor, to a white wire from the control board to a snap togher connector. There it is was marked #13. Another braided high temprature white wire leads from this connector to the sensor. The sensor looked ok, no ash depsits like on a spark plug. I really did not think it was the problem - but was.
Because it looked imposible to get to I gave in and called a repair man($119 weekend rate). Better safe than homeless :*) He thought it was either the flame sensor or controller board and said he could not get a reading off the sensor. He did show me how to take the brakets off and get the channels out to clean the flame sensor.
The large black(in my case) gas pipe from the brown control valve has four flame channels on it. The channels attach to the pipe and lead toward the furnace to a braket. The top braket above the channel comes out with 2 screws, then each of the four channels can be pushed forward (toward the furnace) and off the black gas pipe. Then lift the channel off the lower braket. Repete for the other 3 channels. I thought I would have to move the black gas pip and was afraid to disturb it. You do not have to.
Next, using pliers pull straight back 1/4 inch to remove both the blue and white connectors from the lower braket. Remove the lower braket, 3 screws I think. The braket comes out over(yes over) the black gas pipe. We used some 400 grit sand paper to polish the flame sensor. It was still a dull grey color when done. The technition also bent the sensor to put it in the flame. He used 2 (two) pliers to do this since the base of the sensor is ceramic. Hold the sensor with one plier and bend with teh other. Make a bend half way down the sensor.
Re-assembly: place the lower braket with sensor and spark gap back in and fasten it down. Make sure the 2 wires are attached - the blue one is a high voltage wire - and then put the channels and top braket back in. Channels slip on the gas pipe first and then into the lower braket. Should take 20 to 30 min. We had to cycle the burner a couple of times before the problem disapeard but it has been working better.
Even though I saw no error codes on the conroller board and it does switch from a slow to fast blink, it is getting replaced.
Hope this helps. I am glad somone else posted first. The repair man would not have fixed it had I not seemed like I knew what I was talking about. Hope the directions are ok - I am doing it from memory on a Monday morning :) ha ha ha.