what is a theft module? on 2003 Cadillac DeVille

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one day my car just wouldn't start so I thought it could be the battery, so I bought a new battery but the car still wouldn't start, so I were told that my starter went out , so I bought a new starter but the car still wouldn't start, so I took my car to a repair shop and finally found out that it's my theft module something to do with my security system.
(1) Answer
Basically when you put your key in the ignition and turn it it tells the theft module that it is the right key and it is ok to start. If the theft module does not get the correct signal it will think the vehicle is being stolen and will not start. First thing to make sure is... did you try a spare key you have. I'm sure the repair shop already asked you about this but sometimes the key just goes bad.