My daughters 2004 Titan with 240k miles, runs great and always has. Oil changed regularly. Check engine light came on right before yearly inspection time. ( emissions state). Shows codes 420 and 430. Put in new post cat O2 sensors both side, mass air flow sensors, new plugs and oil change, no exhaust leaks. Tried the O2 sensor extenders, still shows code. I can drive 160 miles at highway speeds and no codes. When I drive in town maybe 10 miles or less, codes come up. Can't get yearly tags until it passes inspection. (Texas). Real time info shows everything is ok. What am I missing?
Don't want to take it to someone that will just throw parts at it-like I did. Thanks.