Glad it was repaired! Sounds like that could have been a bad problem there, had it continued for much longer. Sorry we couldn't lead you there, though popatim did mention FPR.
The O2 replacements and other work you did will not be bad and just may save you a future maintenance issue.
The most interesting thing about this is the fact that there were never any codes or history...?!?!? I still don't know how to throw a CEL without having any record of what threw it. That is perplexing.