Most of the advice here has been right on the money. You are running way lean. If you can make your engine run cooler, you will reduce NOX and it will also run richer which will reduce NOX. Here are some ideas to get you started:
Check your ECT sensor, be sure it's in spec. Putting a resistor in series with it will make the PCM think the engine is colder and enrich the mixture.
Replace your thermostat, to guarantee that it is good and opening reliably and quickly.
Actually, change to a slightly cooler thermostat, but not too drastic or your system won't enter closed loop and the PCM might throw a code.
Make sure the block and head coolant passages are well flushed of all deposits and scale. Use a chemical flush of the cooling system.
Make sure that your water pump is 100% efficient. Many will seem to work, but when you remove them you will find the impeller blades are missing, broke, or corroded away.
Refill cooling system with 30% antifreeze,and 70% distilled water. This gives better heat transfer. Even better, use Redline "water wetter" instead of antifreeze (at least for the test).
Hot wire the radiator cooling fan so that it is always on.
Verify that your O2 sensor is switching high and low at a good rate and in spec. The catalytic converter needs the lean/rich cycles so that the catalysts work properly.
Make sure your radiator is fully operational and not plugged internally. Do this with an infrared thermometer gun--look for cold spots.
Make sure that the radiator fins are clean and clear of debris and the fins are straight. If you have air conditioning, make sure that debris in the condenser is not halting air flow to the radiator. Make sure debris isn't trapped between the condenser and the radiator.
Make sure the A/C is turned off when you go in for the test. You want the condenser cool so that it isn't heating the air flowing to the radiator and engine compartment.
Make sure that the heater core isn't plugged and you are getting heat. If your heater uses a heat control valve, rig it so that it is always on, allowing coolant to flow to the heater core.
A bad MAF can make you run lean.
Vacuum leaks will make you run lean.
A bad MAP can make you run lean.
Good Luck!
