[dogman197]

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.

 

[FishnAlaska]

I would say the next thing to check is the cats, see if they are clogged, seems you have done/checked everything else that could cause those codes

 

[dogman197]

That's what I'm afraid of. Anyway to tell for sure? Is there anything you can ad to the fuel to clear them up? I just need to get by the emission to get it tagged, then I can drive it while working on it.

 

[FishnAlaska]

Secondary cats are fairly easy to check, they should just unbolt, I would think if you get those off, you can look into the pipe and see the primaries with a flashlight. Not aware of anything that would clear it up through the fuel system.