Hey all, Have a different issue that I have not read about on the forum in regards to the AFS code. BTW, you all have helped me through a lot of other issues over the years. So, here it goes: Mastertemp 250 LP, Since the last repairs, heater ran great for 2 years, I just replaced the Thermal Regulator, while doing that noticed a leak, replaced the Heat Exchanger and while in there had to replace the Flameholder and all gaskets. The unit fired right up and ran for 1.5 hrs. Now, after it runs and heats, it shuts off prior to reaching the selected temp and the AFS led illuminates. If I shut off the unit and start it again, it starts normal and goes through the same process of heating for a while (.5 - 2hours), normal operation, then shuts down and throws the AFS code. Inspected all wiring, blower, AFS and all seems to be okay. The temp of the water right now is about 72F and set point is 80. While the heater is on it reaches about 300F which I understand is in the normal range. Anyway, the heater runs fine until it doesn't ;-) I am thinking it may be shutting down because it thinks it is reaching the set point at some time? If I am thinking right, what component drives that and why the AFS code? I dont understand what the AFS would have to do with it.