security

My security light has been coming on recently, and then my car wouldnt start. Said clean key and wait 3 min. Still would not start all night. Started this morning. Went and got key tested. Tested good. Still bought a new key for a spare anyways and the car started. Read previous threads on bypassing the vat. Went to stereo/alarm shop and they said they can bypass it no prob. They tried two or three different times and it wouldnt work. Hooked it back up to factory and now it wont start again. So why couldnt they bypass it on my car but can on others? Is it the ignition or what? Is it something that only the dealership can do? Or can I replace it myself? Im sick of this fuckin car. New motor, new tranny, new computer, and drive it for two days and this shit happens. In case it matters it's a '95 Aurora.