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They run from $20 up to $25K. How much do you want to spend and what kind of features are you looking for?
Myself, I use the Autoxray 6000. Besides codes, it gives you freeze frame data, and real time engine monitoring. It also comes with the software and cable to hook it up to a computer so you can get updates or display data from the unit on your monitor. Auto X-Ray AXR6000 EZ Scan OBD1/OBD2 CD ROM/PC Backup