This is an old revision of the document!
Vehicles newer than 1996 have an OBD-II port that can be used to read/reset Check Engine Light (CEL) codes and various sensor readings.
OBD-II scanners have gotten progressively cheaper and are now available in bluetooth/wifi models that communicate with and display on your smartphone and tablet. They are generally less than $20.
Once you have the CEL code you can goggle it or check it against published lists of codes for your vehicle's make: