A code that indicates worn or inefficient catalyst won't cause your vehicle to run in open loop mode. HOWEVER, are the O2 sensors installed? They are needed to get the proper air/fuel mixture. If they aren't installed your ECM will run in Open Loop (default) mode. If they are bad, that too would cause an Open Loop mode. I replaced all 3x of mine as I was getting bad O2 codes. Not cheap, but last a long time.

I would just get the right Y pipe with cats and be done with it. But keep us in the loop what the issue turns out to be.

 

The 02's are installed, but I'm planning on putting nonfoulers on the downstream ones to stop the code from showing up. As for my idle problem, I explained my new theory for it in my last post. And I decided not to get the y pipe assembly with cats because 1000 was just too much to spend on something that probably wouldn't even change the way it runs.