i need a little help

[slvrsube]

I need some advice before I give up and revert to stock. I have thrown a P0420 CEL and fully understand that the issue has been discussed at length here and on other subaru sites. I just need advice on what to do next. I have a 2002 WRX. I bought an ebay turbo back exhaust (one recommended as "suitable" on here) that included a bellmouth design down pipe. I threw the code almost immediatly. I used the spark plug non-fouler "mechanical" fix but it did not help (with disconnecting the battery and resetting). I read on here that certian down pipes come with an extra O2 sensor bung that helps to eliminate the heating up issue, of which mine does, but placing it there did not help either. I have not tried the "electrical" fix which is using the supressor. I have to mention that I did not install the uppipe as it is not supposed to be necessary at my horsepower goals, the car also came with a AEM cold air intake, and I do not have a tune.
My questions: Is a tune the most logical fix? I would lean to a COBB A.P. for that. Do I need to try and find a stock intake and chuck the AEM? Should I install the uppipe? Should I attempt the "electrical" fix (although I have heard issues of passing emissions)? Should I buy a new downstream O2 sensor?
My car is currently running with expired emission stickers and my gas mileage is beating the crap out of me, as I drive a 100 miles a day. The CEL light is making my brain bleed, and half the time my car runs for crap (at around 2500 RPMs in any gear I sometimes have a lag feeling...it is either this boost creep or a limp mode). Thank you to anyone with solid advice. I apologize for the novel I have written here but if I can't fix this I am going to put back on my stock exhaust and say screw it to stage II.

[KawBoyz11]

Well i cant answer all those questions, but id recommend getting some kind of EM. I run a Cobb AP and love it. Also i used a Cobb downpipe with the non foulers and ive had no problems so far. I did all this about a month ago. So id recommend getting EM and maybe a better downpipe and see where u r then.

[mosc]

any decent EM will disable that code. Also you could use a resistor in place of the sensor which would fix the problem (after a reset to clear it). Repositioning the sensor is really not going to help you much. Those are the only two permanent solutions to this.

[colombian3jo]

i had an 06 wrx, and this 05 sti, that both had turbo backs catless, and i would always throw cels since i didnt have any EM but usually after a day it would disappear and i went to auto zone and had them check the code, and it was always because the car would backfire and the stock ecu would freak out since there was no cat, and it would throw my car in limp mode. it sucks... but i think you should be fine, i ran my car like that for a while, no damage, sooo i would +1 to getting the AP, should get rid of your headache, lol

[IH8PVMNT]

cobb ap

["sub"erman]

Picking up a Cobb will take care of the light. Also, I would suggest removing the AEM intake and replacing with a stock unit. Cold air intakes only screw our cars up.