I pulled codes, nada. I'm tempted to crack the ECU open at my parent's place or even my apartment. I'm replacing the O2 sensors as prophylaxis - the ones I have on right now are Walker universals. The good thing is I live somewhere where I don't need to drive everywhere.

crack it open and post high resolution pics, taken in bright lighting - Yamae and I will give it an experienced perusal!

Hopefully it won't be the cats. I had this problem but I was getting post cat efficiency codes. After chasing it for months it turned out to be the cats. I believe 14 years of junky fuel caused it. Low octane and not as good quality. The car spent its life in Puerto Rico before I brought it back to the U.S. Great car. Loved it. But my 430 has caused me not to miss it. If it turns out to be the cats, aftermarket, although not as nice as OEM, is the way to go. 1/3rd the cost.

There is a procedure to test the cats by monitoring the signal output from the pre and post cat O2 sensors. Maybe Cowboy knows more about this.

if the cats are bad (rare in USA) the post-cat O2 sensors will surely tattle on them!

True. But bad / lazy post cat O2 sensors and exhaust leaks are by far the usual culprit for those getting 420 / 430. That is why there is a procedure to test the cat while on the car.

Found it.

Oh I wasn't trying to imply that if you get a post-cat code, that your cats are bad, quite the contrary, I can't remember when I have seen a sub-300K mile LS400 with bad factory cats, but then I haven't seen the ones in Puerto Rico or Mexico either.

Maybe you have already tried this but I found by taking a 10-15 mile down the freeway and when I get to the smog station left my engine running. Till it was my turn and usually I was next in line in the past if I just drove to the smog station a short drive and my car would pass but not near as good coming off a 15 mile drive. I have over 300,000 miles on the clock and I have only changed the EGR valve and my numbers are perfect .. I also use Arco 91 grade all the time and still get 24-25 mpg on the freeway

While you are barely failing on HC, you are well above where you should be. See that "AVG" number, that's where you should be. You NOx is very low and the CO should be a touch higher as well.

This tells me you are getting a very poor burn and dumping fuel.

Typically this sort of emissions failure is either due to retarded timing, incorrect fuel trimming due to a faulty or failing ECT sensor, O2 sensors being faulty or lazy, exceptionally poor spary pattern from injectors or all of the above.

The ECT sensor doesn't handle fuel trim, the ECU does - it takes information from the O2 sensors, the ECT sensor, the MAF sensor, the TPS, and other inputs and does the fuel trimming and well as ignition timing fine adjustment, to optimize performance, fuel mileage, and emissions, on the fly as the driving conditions change.

The ECU also completely controls the transmission as well as having shutdown power over the A/C compressor in specific situations.

sorry, "due to" not "by"

Installed 2 new O2 sensors and the car's MPG seems to have improved. I haven't pulled the ECU yet.