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BMW M3 barely passed NOx. What can I fix?

I own a 1995 BMW M3. I just passed on NO at idle, everything else was well within range. NO MAX was 695. My BMW produced 691 ppm; barely passing the NOx portion of the smog check. My BMW has 140k miles and has had HG done, fuel injectors cleaned/calibrated, spark plugs replaced, and a recent oil and filter change. My BMW has original ignition coils, CAT, and oxygen sensors. What can I do to be better at next test?
Answer:

Your BMW is a non-EGR valve equipped vehicle. Once all other emissions components are inspected, and either fixed or found to be functioning properly, we may suspect a damaged CAT (Catalytic Converter)… possibly plugged up. The CAT usually becomes plugged after a material breakdown within it normally due high exhaust temperatures and rich fuel. After first inspecting the fuel and ignition systems, an intrusive CAT test and Back Pressure/Vacuum test may be performed. These tests will determine whether the CAT is operational. posted by SmogTips Support

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