'98 Mystique, 2.5 V6, 33k original miles, second owner. Runs smooth before and after CEL came on yesterday. Advance Auto pulled the single code (hope they did it right).

Their printout points to "Catalyst efficiency low-bank 1".
1) Cat converter defective (maybe caused by #2,3,or 4 below).
2) Engine misfire or running condition [Nope, runs smooth as silk.]
3) Large vacuum leak. [Not aware of any, runs smooth.]
4) Engine oil leak into exhaust. [Doesn't use oil & no know leaks.]

No mention of O2 sensor. What are the high probability causes here?

Thanks for your help.