OK, a couple of months ago I began to experience the "Check Engine Light" coming on in my 2002 4Runner. The code was P0420 or Catalyst System Efficiency Below Threshold (Bank 1). I got the light off long enough to get the vehicle smogged today and compared the readings from the smog done in 2012. My initial thought is the MAF is running the vehicle too lean because of the NOx, my second thought was the rear O2 sensor. Just as a precaution I am going to replace both O2 sensors because of their age and mileage (189K). The 1st O2 sensor controls air/fuel ratio or Stoichiometric ratio (14.7:1). Anyway I wanted to hear some of your thoughts. Sorry I can't seem to get the columns to line up.

2012 2014 Limits
HC GPM0.0458 .02313 1.200
CO GPM 2.4523 2.1337 15.000
CO2 GPM 547.7932 521.7096
NOx GPM 0.2412 1.1780 2.0000

You might want to take a temp gun and measure the thermal readings pre cat ,mid cat and post cat to determine if the cat is working so you don't spend a bunch of money on parts you don't need

It looks like most of your levels are actually lower this year with with the exception of nox. I'd say it's probably running fine. If it was getting too lean then HC would begin to rise. Nox could be higher due to an aging cat if it is a 3 way cat. Your emissions tag under the hood will say "TWC" somewhere on the tag if it is. The P0420 would be a secondary indicator that this is probably the case.

If it isn't a TWC then look for carbon build up in the EGR ports.

Thanks guys, I'll check out your suggestions.

Just as Garry said. I'd look into the EGR first. Nox is a sign of high combustion temps. In my many many many many times trough the emissions test with high nox. Only fix was to hook up the EGR. I'd start by pulling it off and cleaning it really well.

Same thing happened on my Camry. It's an older car (96) but the egr got clogged. Just took it off and sprayed some carb cleaner on it and threw it back on. Haven't had any problems since.

It looks like I'm in need of new cats. I have 2 three way cats on it and no EGR if I understand Garry correctly.