After 1996 all cars were made to hook up to an external error code reader. Before 1996 however, cars reported their errors from the computer themselves via a flashing code from the check engine light.
The dreaded check engine light comes on while you are driving only to signify that there is a problem. The specific error or failing part could be one of many different things that the computer senses for. You can bridge a set of wires in your car and send the cars computer into its error reporting mode so that you can find out whats wrong with your car.
Again, this instructable is only useful for people who have cars that report engine trouble via a flashing check engine light. This should be true for many cars made before 1996.
Here is a short video explaining the code deciphering process. For the full instructions check out the following steps.