..with a check engine light on. My car has been in the shop for the past two months while this mechanic took his sweet little time doing a head gasket job. Now when the car went in it did not have this message...so I'm not sure what happened between then and now. Also it is to my understanding that this vehicle has multiple catalytic converters. How do I figure out which one is faulty.

 

CATS is not cats LOL

CATS is C.A.T.S.
which is Computer Active Technology Suspension, i.e. it's a suspension-related message.

Maybe he unplugged something, such as the wiring concerned? E.g. the connector(s) on top of the dampers (shock absorbers). Take the car back. If you want to take a quick look yourself, look first under the front wheel arches - see the big springs going upwards and the rod things inside the springs. Now look inside the engine bay at where those are fixed and there should be wiring from the top of each. Those wires are the likely ones, or somewhere they disappear to.

 

Ohh okay. Lol