Some quick thoughts:
- Your car's 24 years old. If the downstream O2 sensor's never been replaced, it's probably due. The oxy sensors run in a hostile environment and eventually all go.
- I am on the fence about maintenance replacement of O2 sensors. On the one hand, the car will run better if you do. On the other hand, the cheaply made (NOT necesarily cheaply PRICED) O2 sensors are awful. I did a maintenance replacement of my upstream O2 with an ACDelco sensor and much to my surprise had problems. Replaced it *again* with a Denso I got from Rickw as NOS (he had on hand for his now gone Riv) and problem solved. Point is, as always, watch out for counterfeit parts. I wonder if the Delco was counterfeit or just bad new.
- Especially if you've never really done so, check your fuse box for corrosion from water intrusion or just age, especially the fuses to devices that are sending codes. I used to check my fuse box a lot, never found water intrusion but did find a fuse that would not conduct although the failure was not visible (almost all the time they are obvious on inspection). While you are at it, pull and reseat the fuses for those devices; look at the connectors. There should be no problems, the main point of doing this is to get familiar with the thing.