OBD II for 1996 upwards...a simple scan tool from Harbor Freight will get you started reading codes. After that, the sky is the limit. Be sure to get one that is compatible with the later protocols (CAN bus) so that you can use it on other cars.
Actron makes several versions that do more...including freeze frame.
The Big Daddy to diagnose and TREAT your car is the DRB III...available from Tech Authority for about $6,500.00.
Then there was the StarSCAN (free standing...no longer supported) and StarMOBILE (used with a PC)...though one still needed a DRB III for thr Tire Pressure Monitor System found on 2003-2010 Vipers.
Later Vipers (2008-10) required BOTH a DRB III and another tool called a wiTECH ($2,600.00) plus a PC... though the latest wiTECH software now turns the wiTECH into a DRB III emulator.