Scan tools

What years are you working on? The more you pay, the more you get. Pishta mentioned OBD codes (for 1996 and newer vehicles). There are generic codes that are the same for all manufacturers and Manufacture specific codes that are well, manufacture specific. The cheaper scanners probably won't read the MFG. codes or you probably won't have any bi-directional capability. (to operate/cycle things like egr valves, canister purge valves and the like.) Get what you can afford.