One cat. Not Cali. Plenum gasket was replaced a few months ago.

 

Was it replaced with an alum belly pan, or another steel pan?

If you bang on the cat, does it rattle?

Also, check your pcv. If it has failed, you can get oil blow-by into the intake, which then acts just like when you have a leaky plenum. Probably need to do that tune-up too.

 

PCV is newer and good.
Knocking on the cat sounds like there's a brick in there but not a box of rocks.