I don't think leaving the ecu in diagnostic mode would have smoked it. Not sure if that is what you are trying to say or not, but you definately have another problem. My car when I bought it was in diagnostic mode for at least two years prior because I was fixing a bunch of stuff and when I pulled the ecu to check codes it was already in diag mode. I talked to the previous owner and he said he never even knew or had the ecu out for any reason.
