Failed emisons inspection

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I just failed the smog inspection in CA. 1994 940 wagon (non-turbo) w/254,000 miles. The part of the test that it failed is the NOx portion at the 15 mph speed but it passed the 25 mph. The max emissions allowed is 556 but i measured 686. The catalytic converter was just replaced 2 years ago, oil changed 2 weeks ago, plugs last done at 237,000 miles and the "check engine" light is not on.

ANY suggestions?

High NOX is probably a vacuum leak somewhere. Your 1994 doesn't have the EVAP test of the 1996 on (I think) so my guess is a vacuum hose is split, leaking, or loose somewhere