NOx reading high ....engine slightly lean but not lean enough to cause misfire. NOx goes down when misfiring due to colder cylinder temp. Vacuum leaks (unmetered air entering intake system) or low fuel pressure will make it lean. You can test the Catalytic Converter.
Performing the Catalyst Delta-Temp Test.
To perform this test you will need a infrared temperature gun.
http://www.toolprofessional.net/Mastercool-Infrared-Thermometer-with-Laser-in/M/B000TM7HXC.htm
Disable the AIR System, if equipped.
Run engine for two minutes at 2500 RPM to fully heat the exhaust system.
Engine off.
Disconnect and ground one spark plug wire from each cylinder bank.
Start engine and run at 1000 RPM.
Disconnect the IAC solenoid.
Measure the surface temperature of both the inlet and outlet of each catalytic converter using an infrared temperature probe.
Turn Engine off.
Compare the difference in temperature between the inlet and outlet readings.
If the difference is 28°C (50°F) or greater: The catalyst is operating correctly. Reconnect spark plug wire(s), IAC solenoid and AIR system (if required). Perform PCM Reset to clear any DTCs set during testing.
If the difference in temperature between the catalyst inlet and outlet is less than 28°C (50°F): Testing indicates the catalyst is not working. Repeat test to verify results. If the temperature difference is still less than 28°C (50°F), replace the catalyst. Reconnect spark plug wire(s), IAC solenoid and AIR system (if required). Perform PCM Reset to clear any DTCs set during testing.