It’s the first one to roll in your door-a late model with OBD II, the check engine light brightly illuminated on the dash. The owner may or may not have some performance complaint that goes along with it; but you know this system, like engine control systems for some time, can compensate for most problems, can mask what would otherwise be driveability symptoms. You find the standardized 16-pin connector along the lower dashboard, and you have a new scan tool that can pull the new codes, but how do you use the system to find out what’s wrong with this car so you can repair it?