I have a 04 Rubi that I'm wanting to replace the cats on. My question is: I know that I currently have the 2 mini cats, but I do not see a larger rear cat underneath towards the rear. I have a feeling that the PO cut off the big one and welded in a straight pipe. Can someone tell me if replacing the current set-up with one of the direct fit units listed on rock auto that has the larger rear cat will cause any issues?? All of the O2 sensors are close to the mini cats, so what's the point/function of the larger rear cat I see in the pic? Any help or advice would be appreciated as I'm needing to get this done ASAP to pass emission testing before March 1st.