P0431 front cat?

GTS with 86k.

Getting the P0431 code. If I erase it with the torque app it probably takes about 100 miles to come back.

The 9G manual doesn't seem to touch the subject but alldata says step 1 is to replace the left cat.

This is the front cat correct? Also rockauto seems to be the only place that lists one. It's around $400 with the exhaust manifold, they don't list a cat by itself. Is there a way to get the cat by itself and save some money? I haven't looked under the heat shield but I'm assuming it bolts off. I'm also familiar enough with working on exhaust to know how much of a bitch it is to get something like that off without breaking a bolt.

Rockauto lists a 49 state and a OBDII compliant "OEM grade" one for about $40 more anyone know what the difference is? They do not emissions test in my state but I don't want the check engine light on all the time.