4kcsq Failed Emissions

[Altieri, Anthony (MBFF cs)]

My '87 4kq recently failed CT emissions.  CO was 5.66. (pass is <1.08).  HC
barely passed at 161.1 (pass is <168.0).  Can someone please lay out
possible causes for this type of failure?  I know it can't be diagnosed like
this, but a list of possible culprits would be helpful.  As far as I know,
the O2 sensor and Cat are suspect, but beyond that, I don't know.

TIA,

Anthony.

PS.  The car also failed because they said the fuel cap had a bad seal (???)
Any thoughts on that?