We just moved to Maryland near Washington DC. Lived in Boston before. The '93 940 wagon with 160K miles passed Massachusetts emmisions (dyno with tailpipe sniffer) last fall. As far as I know Mass test is very strict (just like Cali?) and the car passed with flying colors. Now we registered it in MD and required to re-test emmissions. All readings but NOx (oxides of nitrogen) are well withing specs. Very low HC (burns no oil), almost no CO to speak of (catalytic converter is working), but NOx is high at all speeds up to 60mph. They give you these neat graphs here from the test. Anyway, my question is what should I look at? I have a long time to re-test (until November, I think). I am planning to look at the uisuall suspects: plugs, air filter, etc. I will run fuel treatment through the car as well as a few tankfuls of premium (Ouch!), but can anyone explain to me what high NOx emissions mean? I understand HC is oil related, CO is fuel muxture, catalytic converter, etc. but NOx I have no idea.

Would appreciate some info on this.

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Vladimir. '98 S70 base, 5-speed manual - his, '93 945 - hers,