Oxides of Nitrogen are formed when Nitrogen gets extremely hot and bonds with oxygen. When running correctly the engine uses the EGR system to put some inert gas back into the engine to keep combustion temps down and prevent nox emissions. You might want to try the timing trick or try checking out the egr system to see if everything is intact. The engine could be running a little lean. Where did it fail? At idle, cruise or somewhere else, did they tell you?

CHRIS
98 Amigo, 92 Pup

need a pickup 1st gen fuel level sender