are excellent for the car, don't overanalyze it. Your Co and HC are very good. Run some diesel fuel injector cleaner in there as I suggested and see how it goes. The state of Virginia has actually made it harder on these cars, if you go back to your tests 2 and 4 years ago you'll probably see the NOX limit has been lowered meaning the car must perform better.

High NOX is not 700 or so. It may be for the state of virginia but isn't for an older car. I've been amazed by my own propensity, and that of many folks, to react to a number that is at or slightly above a state limit. The state limits are somewhat arbitrary and do not take into account age of the car, age of the catalytic converter, etc. I've studied the equations, relationships, etc between NoX, oxygen, etc etc etc. I can tell you that you are on the path to hell here if you overdo it-- I did that. If you sense the car is UNDERpowered then you have a reason to go in there and determine if the cat is clogged, etc. Of course check for vacuum leaks but I don't see that as a likely cause here per my other post, but what do any of us know really in the end since we guess with an esoteric thing like NOX value.

If you are experiencing less power with the car, then you probably have a clogged cat. I don't know if your car has pre-cats, but they can clog. If your power is good and everything else is good, run some cleaner in there and leave it alone. Trust me, you're on the way to a very big litany of stuff that may result in a better car in the end (maybe more power) but great expense as every mechanic or you go in there and try to guess the thing out. If it really bothers you, other than cheap and easy things you can do, the other thing is to put a Miller cat AND pre-cats on there costing you $1200 to $1500 for someone else to do it or I think about $600 parts plus whatever labor you can find-- that's a quote for another MB (mine).