I would like to add that error codes E05 and E06 will not automatically clear. You should cut the power to your heater for a few seconds. After I reset the breaker, E06 was cleared and the heater started working again. It's a simple 5 second process that may save you the time, effort and frustration of replacing the flue stack sensor. I wonder how many folks have replaced their sensor when a breaker reset would have fixed the problem. I was in the process of ordering a new one when I found out that you have to reset the breaker to clear the code. When I replaced the sensor a few years ago, I shut off power to the unit while servicing it. Interesting. Hope this helps.