You have replaced a couple of tstats in the past few years? THAT is NOT normal. Tstats rarely fail. So, your problem was there prior and now it is there for good. When the fan comes ON, The sail switch closes and sends power thru the Hi limit to the control board. Now, without the brand and model, we can only guess now. It is not uncommon for certain Atwoods the wire going to the hi limit switch touch the burner casing and melt and short. THAT would be my first thing to look and verify. Gas Valves shorting out? I have not seen such a thing in over 25 years and I have worked on thousands of furnaces in that time. Possible corrosion on the Control board. Most furnaces have a circuit breaker to prevent shorts from burning out tstats and such. Doug
