Figuring out what's going on

[theolsisterinlawssubby]

My sister-in-law bought a Subaru Crosstrek a while back, had a new engine but started getting p0420 codes. Well, she didn't do anything about it and it blew the engine the dealership put another engine in it as it was still under warrantee. But even with the new engine it still didn't have much power and... I then replaced the cats for her, and it still had a p0420 so replaced the o2 sensor. Now the p0420 is off but we now have a p2196 which is the o2 stuck rich. Sometimes with the AC on the power loss is immense. No backfires. I cleaned the MAF, listened to the injectors for one to possibly be stuck open. I can't find a fuel pressure testing nipple anywhere. What should I be looking for next?