Had P1100 and P1151. Now getting P0420.

[ldyoung71]

I originally had p1151 and p1100. I replaced the bank 2 sensor 1 O2 sensor and cleaned the MAF sensor element with maf cleaner. I drive for a few days and the CEL came back on today. I drove to AutoZone and they pulled a p0420 this time. I have reviewed this site for the error and come up with several things to do.
1) PCM flash?? How do I do this?
2) Mileliminator. If I do this will I be able to pass emissions tests? I assumed they will know if i am bypassing.
3) Clean UIM.
4) Replace sensor in bank 1 sensor 2 O2 sensor. Where is this one located? I have seen conflicting answers.
5) talks of IAC and TPS being culprits.

Based on my symptoms and what originally was my issue(p1151 and p1100), what do you think should be my plan of attack? Are there any experts out there that can diagnose the issue based on my order of receiving the codes??

 

[gomach1]

I just installed MIL eliminators and had it inspected a few hours after and it passed no problems! Before that though, i cleaned the UIM/LIM/EGR, replaced spark plugs and all for it to run better, yet still throw the P0420... That sensor is the furthest one down the exhaust, on the backside of the engine. Post-cat sensor... Id do the MIL Elim and be doen...

 

[ldyoung71]

OK. I guess that answers one of my questions. I was wondering if the emissions people could tell if the MIL elims were installed. If they won't be able to tell then that is fine with me.