Coleman Evcon furnace... Only service techs need reply?

I am a service tech and I'm not quite sure what to make of this situation. I recently had a customer call saying their furnace is not working. On arrival I noticed that the diagnostic light wasn't even on. On inspection I found out that the circuit board was bad, because you could see the burnt resistor on the board. I replaced the board with an exact replacement. The furnace fired up just like normal, and everything worked properly. 3 days later the customer called back saying the furnace quit working. When I checked it the second time, the diagnostic light was flashing a rapid constant flash. I checked the schematic to see what a rapid flashing would mean, and there was nothing listed for that type of error code. I went on-line looking if there was anything that would describe that kind of error code, and I came across a site that indicated that it meant a reversed polarity problem. I tried reversing the polarity, but that didn't make any difference. I assume the board is bad in some way, but if anyone would have any other suggestions, it would be greatly appreciated.