So, I have an '05 RX-8 that lies at 122k miles atm, changed the motor recently with one of lower mileage. Car came with certain mods, I added some bolt-ons to the list, but it's nothing fancy. I've been through a troubleshooting stage where I tried getting rid of all my CEL codes so I can pass emissions, and it worked. Car was running fine (other than a dying transmission and such).

After passing emissions I decided to put my resonated midpipe back on that came with the car when i bought it, but I put a CAT on so I can pass emissions. Keep in mind, at this point, no CEL or anything, all is running well, all the emissions monitors ready. 200 or so miles into the midpipe, I was expecting a catalytic efficiency CEL, but NOTHING! To my surprise even the Catalytic monitor was ready. Then I started getting a CEL on and off that had to do with the P0139 code, O2 sensor circuit slow response (bank 1 sensor 2). Some said it's a common code if you have a midpipe or things of the sort, so I ignored it. 400 miles into my midpipe and now I'm getting the P2096 code, Post Catalyst Fuel Trim System Too Lean (bank 1). It triggered after a 45 minute drive in really hot weather on the highway.

Could the 2 be correlated somehow? Anything to worry about? I have an AEM cold air intake, if that helps at all to track down what's going on. I was thinking maybe cleaning the MAF sensor and the other sensor on it would possibly do anything? I know that running lean means that not enough fuel is getting into the system and it's pretty bad for the engine. I just don't know if the O2 sensor has anything to do with it. I've tried some pre-mixing, very low ratios, and the next 2 fillups I would leave it alone and throw in some fuel injector cleaner.

 

Did you ever diagnose the issue? I’m having those same CEL codes; the p2096 and p0139.