Yes, I understand that the threshold for passing these emissions test is 'politically' set but why would one of the indicators give poorer results than the other two? It has been almost 40 years since I took that horrible 'thermodynamics' course in 2nd year college (I didn't understand it then but managed a pass)..... Surely if combustion is (relatively) complete, resulting in few unburnt hydrocarbons & little CO compared to CO2, then oxides of nitrogen should be at the expected level also, not in excess..... As you can tell, my understanding is feeble.
Our 96 D1 has only 33k miles and has, I think, a very clean engine - heads removed and decoked 18 months ago. Perhaps I should up-date the plugs and wires anyway?
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