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.