2003 grand am will only start if I reset the ECU

Car runs well, idles, revs and drives, security already disabled, fuel pump kicks in...WHAT CAN BE WRONG!

OK so I got this car super cheap because it "suddenly started running poorly", when it was dropped of the owner boosted it and it started right up. I went to work trouble shooting the cause of the rough running and discovered that the main vacuum line wasn't hooked up. GREAT...so I boost it and it starts right up and runs awesome. So now all I need is a new battery....right?....Wrong....

I get a new battery and install it and it starts, runs great and I'm pretty happy and proud of the great deal I got.

Next day I go to start it up and it just cranks and cranks but will not start...I fiddled with a bunch of things...checked security issues with these cars and spend a few hours trying to "relearn the keys" only to discover that there is no chip in these keys and the resistor patch was already put in. For some reason during this process I disconnected the battery cables and after waiting a few minutes I hooked them back up, and the car starts right up, a little struggle to start but start it does. It would start right up for me after 1-4 hours after the battery was disconnected, but it will not start after 10 hours of the battery being disconnected. I disconnect the battery cable....wait 5 minutes and away she goes....10 hours later...just cranks and won't start....What can be the problem? Why will it only start if I reset the computer? I have it at the shop right now and have stumped more than one mechanic.

very perplexing!!

Check your CEL code with scanner. Unless you know for sure the CEL light isn't broken or disable.

Thanks for replying...just got word from the shop...appears that it was a 10amp a/c fuse was the culprit replaced that and it started right up...not sure why just yet but someplace to start anyhow.