Hey all, I'm troubleshooting my 2005 Nissan Frontier 2.5L with a 5spd transmission and 180k miles. Codes are P0420 catalytic, P0507 high idle, and P0302 cyl 2 misfire (although this one hasn't shown up in a few days). Actual issues are stuttering during acceleration as well as high/funky idle (stutters and sometimes when fully warmed, idle oscillates between 1k and 1.5k).

I have replaced all sparks, replaced PCV valve (was literally sheared in half), cleaned MAF sensor, and looked for vacuum leaks (none found but also LTFT is close to 0). None of these have helped.

I am using a ThinkOBD 100 to do troubleshooting and collect data while I drive. LTFT is looking fine, and the truck goes into CL (although sometimes it says "OL-[ CL" and I don't know what that means), I noticed the upstream O2 sensor does not bounce between 0.1V and 1.0V like it should, it stays steady at 0.3V or 0.35V (even in CL).

What could cause the O2 sensor to stay steady like that, even in CL and fully warmed up? The sensor reading used to bounce around when I checked it out during summer.

Thank you!