Thanks Big Jim,
Although, due to the rains, I have not check the O2 sensor (l'll do it with the Autoxray) I have found some other things. Does anybody know what does it means "OBD Exempt"? the emissions sticker has that. I also learned that NOx is created by high temperature conditions in the combustion chamber and that engines have the EGR valve to control the temperature by recirculation of exhaust gases. My Jeep does not have one ! & the timing is not adjustable (it can be retarded by 2 degrees only, by law).
I am lost, any information will be appreciated.
Thanks