Stock cat (or equivalent) for STS, otherwise you get bumped to STX.
E. Catalytic converters may be replaced by aftermarket units. Replacements
must:
1) Be certified for use in that vehicle application by the manufacturer or reconditioner,
2) Bear correct EPA-mandated labeling,
3) Be of the OE quantity and type (i.e. oxidation, three-way, etc.)
and
4) Be used in the same location(s) as the OE converter(s). This does allow for high performance replacements, provided they meet all restrictions herein.