OK, I have done some research. I would like everyone to know that I am sorry, and that I would like to you to read this post, for I fell that it is far more accurate than my previous posts.

There is no such thing as a NOx sensor.

A knock sensor, is usually found on cars with high compression engines. The knock sensor senses the vibrations of the engine block due to predetonation of the fuel in the combustion chamber. The knock sensor tells the car computer (EMC) to retard the timing of the engine to avoid pinging- which causes serious damage.

When the fuel mixture is too lean, it becomes very hot, and everything about what I said before: "because of the lean fuel mixture which makes the bonds of N2 which is regularly found in air at close to 80% of air to combine with O2 to form N20, NO, N2O5, and other NOx mixtures. Instead of N2 being a non-reacting molecule passing through the engine, it reacts with air at high temperatures to form N20, NO, N2O5, etc. Basically N2 + O2 ~~~> NOx emissions, which is an endothermic reaction (requires heat to react)." The EGR valve lets some exhaust gas back into the engine to lower combustion temperatures to reduce these types of emissions. The knock sensor has nothing to do with this.

I spoke with my brother about knock sensors, he said if you were to hit the side of the block with a hammer (not endorsing this), and the the engine started running bad, then you would know that the car has a knock sensor because it sensed the vibration and retarted the timing.

I spoke with a Mercedes Tech about the SL600. He said if the car is pinging, check to see if you are running premium. You said you were, so thats probably not the problem. The next thing it to make sure that the knock sensor is working-retarding the timing.

For Gilly,
Ricers use Nitrous oxide which is N20.
N20 Nitrous oxide (old naming) Dinitrogen monoxide (new naming)
N0 Nitrogen monoxide
Nitrous oxide is used by ricers to provide more oxygen for the fuel to burn, but consequently they dont care about the emissions of their car when they are racing.

Very sorry for the poor information. I hope this has helped clarify the subject and help you know what to do with your SL600.

Regards,

__________________
Current Stable:
1994 S500 v140, 210k miles, white with grey.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold (rumor has it, that it has 750k miles on it now)
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 "300E" Amg Hammer 88k mi sold (it was really a 200, not even an E (124.020)
1992 500E 156k mi sold
etc.