Aftermaket exhaust, PO420 code?

[AlaxyGalaxy]

I swapped my Toyota midpipe (2004 Echo HB) for a used aftermarket OEM-style pipe (muffler connection was rusted), and within 50km, the car threw a PO420 code. I cleared the pending and the actual codes, but after my 15km drive to work this morning, I have another pending PO420.

I'm not sure of the brand of the exhaust (the muffler was a Walker), but it was said to have 3000km. I believe it since I removed it myself and the spring bolts on the manifold were easy as butter to remove.

I kept my original O2 sensors, but I didn't disconnect the downstream sensor and the wires *did* get pretty twisted when I was unscrewing it, despite my being very careful.

I put a lot of anti-seize compound on the threads of the O2 sensors, but I was careful not to put any on the sensors themselves.

I installed a Borla center-exit muffler at the same time. It sounds great! at least 20 extra HP! Maybe even 50! LOL



What are your thoughts? Are aftermarket cats as efficient as Toyota cats? I still have my original midpipe and I guess I could chop it up and replace the resonator with a Walker 54657. What about O2 sensors?

Thank you.