01 solara 5SFE PO420

[bigbodychevrolet]

I have a 01 solara with 2.2 about 135k and a reoccuring PO420. I replaced the rear cat and now the same prob exists don't know what it is now exactly. the a/f sensor, rear o2 sersor, or precat, exhaust leak? I don't hear a leak, the data on the scan tool only has 02s12. which bank or sonsor is that? Its only reading one. when i do 02 sensor tests it can't find any info for the rear sensor. the voltage it does read on the 02s12 is very erratic .67 then to .04 in seconds i don't think this is normal. i'm thinking of just replacing all three the sensors and precat. I was also wondering if i could replace the precat with a header. anyone make one thats worth having? would a header keep my light on? the light really bugs me. I'm getting about 17mpg right now constantly. same driving habits have always had was getting around 24 city keep track of that stuff. any help would be appreciated. thanks