Help, Failed Smog NO too high at 15 mph ONLY BUT passed at 25 mph-

I have a 1999 Chevy Cavalier
My 1999 Cavalier failed smog at the 15mph test. The CO2, HC, CO were all low - not even close. But the NO at 15 mph failed. The 25 mph test had no problem. NO @ 25 mph measured 144, Max allowed was 746. So well below. NO @ 15 mph measured 752, Max allowed was 459. So It failed badly. Any suggestions? I first thought the Catalytic converter BUT, it passed easily at 25mph. So maybe the O2 Sensor? Any suggestions? Thanks, I drive it mainly on freeway to work and back. I drive alot of miles.

Assuming you mean nox., the egr system needs to be checked out.
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