Carrier 58ZAV with code 33 and is not heating, blower is off, flames are off, while thermostat says "HEAT ON", set temp is 76 and actual temp is 72. Stays this way for hours while ambient room temp drops and thermostat reads actual temp at 70. To fix it, I have to turn the thermostat to OFF then turn it back to HEAT. This clears the code 33 on the furnace. Then the "HEAT ON" will read because the ambient temp is 4+ degrees below the actual temp and the furnace will fire up and then the blower will come on and the heat will work until the set point temperature is reached at which point the thermostat will change from "HEAT ON" to "HEAT". Then the room temp will slowly drop to below the set point by 2 or 3 or 4 degrees, the thermostat will change from "HEAT" to "HEAT ON", and the furnace will not run (not heating, blower is off, flames are off) and will flash a code 33 and will not heat and we are back to the starting point again. The flame sensor has been cleaned and sanded down slightly. The control board has been checked and it checks out a-okay. The thermostat has been replaced with a simple no-program thermostat. Because of the code 33, could it be that the open flame rollout switch needs replacement? The code 33 never changes to code 13.