If you search on the forum, there are a couple posts that detail a way to read the error codes w/ a home-made code reader (and how to make the code reader).

Also, as was mentioned before, I want to stress the importance of the coil pack, especially if you are running the original. There is no way to test the coil, apart from replacing it. If you plan on keeping the car, you mind as well replace the coil, as they don't last forever. In my case, the coil was the main issue and calmed the roughness down a lot.

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87 300E
202k on the clock