if lots of miles....have a basic tune up done.....by the way
O2 sensor, EGR valve, PCV etc should be checked and or changed
at around 80k anyway.
If the HC's are high at idle, have the EGR checked...
if CO % is high at idle check O2 sensor and air filter checked.
sometimes airby pass valves get stuck shut and cause a rich condition on EFI cars as well. (they are usually located on the throttle body.
NOX is would be lower if rich and CO would be too high. Assuming your jeep is FI, back the timing off. NOX is higher when cylinder pressures are higher or the cylinder is hotter. This can happen when timing is too far advanced, lean conditions or ERG valve is not opening correctly. If retarding the timing does not help, make sure your ERG valve is working. Also might help to run a colder spark plug. Good luck
A rich mixture will NOT cause high NOx.NOx is caused by high combustion temperature.Check EGR for proper operation.Apply vacume directly to the vave at idle.If the passages are clear the eng should stall or at least come close.Check plugs for exccesive gap(ware)and check timming.Also check fuel presure.And make sure the eng isn't running to hot.If all the above are OK the problem is probably the catalitic converter.The work around for this problem would be to retard the timming advance.In a Ford you would pull the Spout conector.On a Jeep do whatever you would do to check base timming and leave it that way untill after the test.
I"m a certified emmissions inspector and tech.
The last two posters are correct. In general, a rich condition will not cause high NOx emission. High cylinder pressures will cause high engine out NOx. A lean exhaust condition will cause reduced catalyst effiency for NOx. This could be caused by an exhaust manifold leak.
As the previous poster stated, check your timing and EGR valve. One of these are probably the cause.
Thanks for the insights-this car has electronic timing so no joy there. I am going to check the egr this evening. I am having the catalytic converter changed out since it rattles anyway-probably will fix the problem.