I have a Lennox furnace that has what appeared to be a flame sensor issue.
When the thermostat calls for heat, the furnace starts up, the HSI heats up, the flames start, but then the gas shuts off after about 3 seconds and the cycle starts over. This cycle may run once or twice and then the furnace will stay on until the thermostat is satisfied. Occasionally it will cycle enough times to lock out the furnace.
I thought it was a dirty flame sensor, so I took it out and sanded it, then reinstalled it. The furnace worked great at first when I turned the power back on, but when the second heat call was made, the problem showed up again. I worked on the furnace twice and both times, it worked great when I first turned the power back on, but the problem reappeared once the second and subsequent heat calls were made. Could the fact the furnace works when it is cold but not so well when it is warm be a clue? Any help is greatly appreciated!