NOx forms when cylinder temps reaches 2500F and higher which indicates a lean condition. A lean mixture will produce oxides of nitrogen since carbon monoxide and hyrdocarbons are down.

I would have the lamda adjusted and the oxygen sensor tested out. Then I would make sure the fuel delievery system is delivering a good amount of fuel.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)