My guess is that is one of many ways that can cause you to fail the nox test. Perhaps take the car to your mechanic or even midas and have them tell you if it is the original. If it does have a cheap converter (non mercedes) on, you can probably get by by just installing another midas converter.

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.