The main fan does not turn off after the burners stop. Sometime it works and other times it does not. It will also turn on when the buners are not lit and run for long periods. I have been manually turning the furnace off and on. I have replaced the ingnitor and kept it very clean and that does not seem to fix it. Can you please point me in the right direction to fix this problem, I am thinking about trying the fan limit switch.