After market electronic anti theft device called an ATS which no longer make the little chip that goes in it started messing up until one day it just quit starting so now the pass key fault flashes an

Do you still have an original chipped key programmed into the car's passlock system? If not may mean a trip to the dealer.

You must be thinking of the VATS system. There are bypass kits out there. Google VATS bypass.

This is a very common problem on GM vehicles. The security system is known to fail. Basically when you insert your key, the system checks for certain OHMS resistance. If the system cannot get the correct signal from the key, it assumes the key is incorrect and goes into SECURITY lockout mode. (prevents hotwiring) This has been happening with GMs for a very long time. There are ways to bypass this now. Read through this page for info, solutions as well. ​ ​http://www.diy-gm-security.com/