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Well, it's pretty simple and inexpensive. You can do it with just a piece of wire and count the flashes on the dash. The other way is to buy a EEC_IV code reader. They run around $30 and also include a book with all the codes listed and a detailed description on how to use it.
The code reader can be found at your local parts store. One thing about it though. It is just a light that will flash anyway. Why not just use the jumper wire and watch the light flash in your truck. The check engine light. You can also hook up a multi-meter and count the sweeps the needle makes. Just use your Chiltons or Haynes manuals for the code meanings. If you have an Autozone in your area, then just take your truck down and they will test it for free.