'91 80q emissions

[Brandon Rogers]

My sister has a '91 80q that has had some serious work done by the dealer
recently because it failed emissions a few months ago.  Other than failing,
the car was driving great.  After some parts were installed to fix the
emissions, other problems popped up--stalling and erradic idle. Now it still
occasionally stalls and it still failed emissions-barely.  HC was .5 where
the limit is 2.5--Ok in that category.  But CO was 22 where the limit is 20.
Any ideas?  02 sensor?  TIA!

Brandon