On the 90 & 92 both which had high NOx and very low CO I replaced both the
Y-pipe & the O2 sensors. Both had over 200k and they both passed on the
retest. Didn't try one then the other as I didn't want to waste time going
back to the DMV a third time. The O2 sensors probally degrade over time and
are not as accurate as when new.
Ken