Exhaust issue. codes thrown.

[SERUM]

I have a 97 240.

Something is rattling around in my exhaust. Based on what i've read here the cat under the car is hollow, with the one in the downpipe being the actual cat. The FSM was no help cause it didn't label what was what. I purchased a catback, but don't wanna have it installed yet if I need a new cat.

As for the code issue, it always came and went so I always ignored it. I think it was some other exhaust related code, but now it's a p0400 probably from the exhaust being clogged and my ignorance.

Could someone explain this whole fake cat real cat ordeal? Or offer any input?