hey scotty this is huge question? i have an 03 impala 3.8L 1 year ago today it stopped starting and running... the engine cranks until car battery dies.. i ended up taking it to a machanic shop and they had it for 7 months.. they had no clue what is wrong with it.. i replaced every sensor in the car.. relearned it and started but once u shut the car off it will not start again..i had to relearn it again and i cld drive the car all day ... no issues what so ever while the car was driving around.. then turn car off then wont start.. it just cranks til it dies... now i cant even relearn the key.. meaning put key to on position for 10 mins and do it 3 times... well someone said i needed a new computer well i went to the junk yard and got same year computer and the matching pcm.. then relearn and got nothing .. radio works lights come on but wont turn.. so i put old back in and well same issue .. got nothing .. my car has been sitting for over a year.. my wife and i share a vehicle and it wld be nice to have a 2 car again.. and if u cant help me then i will have to see if the chevy machanics can fix it but have a feeling that it will cost me a fortune to do it that way.. thanks

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realize GMs have crappy computers, you get a used one from a junk yard, odds are it's bad. Your car has the classic signs of computer and or wiring system failure. I would start with a computer from a company like NAPA that can sell you an already programmed one and if necessary, pay a locksmith to do the coding for the anti theft system. Good luck