These models use the OBDll system to read trouble codes generated by the on board computer. After you repair or peplace broken components you should clear the codes. It would do you good to purchase Haynes repair manual (about 20 bucks). Also the passive theft detterent system will disable the engine from starting If the steerig wheel is tampered with. That component used to be located behind the glove box on earlier models. If you had an aftermarket alarm installed check the valet switch. See If the LED is lit when you press the button. These OBDll diagnostic tools can be bought for a wide price range, which is directly related to what functions the tool can perform. I paid about $80 on ebay for one with a money back 30 day warranty. You really need one If you expect to service your own Cadillac these days.