How to Read Codes From Your Check Engine Light (For older cars)
You can pay a mechanic a bunch of money to read your car's error
codes when your check engine light comes on, or you can grab a
paper clip and read the codes yourself.  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 decipher