I keep getting an E05 code. I replaced the stack flu sensor and it worked for about two days. It started giving me the air code again so I replaced the sensor once again. This time it is not working at all. It turns on and ignites and runs for about 15 seconds then it shuts off and gives me the EO five code. I check the wires, connectivity on both the unit and the sensor but it all looks fine. What could it be??