There are three reset sensors on this unit. One on each side of the heat exchanger and one in the heat exhaust flue. The unit kicks off after about 5 minutes operating. I have to reset the sensor to get it to come on again for it to do it again. I have cleaned the sensor itself with sand paper. Filters are all clean and air flow seems to be good. I would suspect that the sensors need replacing. I need to find a source for purchasing.