That 231 code is "compensating for mixture, too lean or too rich" and is pretty vague at best as I recall. I think it could be a bad fuel pressure regulator, vacuum leak in any intake hose, and a few other things. Start by pulling the front hose of the FPR and checking for gas smell or drip. Not supposed to be there, diaphragm failure. Easy to replace. Anyway, start there. There are other faults that produce the code, Dan Page and I once had a list we talked about, but I did not write it all down. Dan will have some good ideas.