Open the hood and look on the passenger side of the engine compartment. There is a large black computer box, and hanging off the front of that box are two small black boxes. Lift the lid on the left box straight up. It has a probe in the lid. Then follow the instructions 1/2 page down on this link:

https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

This is for the "A" box. The "B" box contains other stuff, like the instrument cluster, power seats, etc. You want to get the codes for A-2, so stick the probe in A-2 and read the codes.

The autoparts store code reader should also work, but it will give you generic codes for the engine and emissions only.
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Keeping it running is better than buying new