I have a 2002 C320 that has driven great up until now. The check engine light came on two weeks ago and I took it to an independent mechanic who specializes in Mercedes. The car was idling and vibrating at stop lights. The codes said Banks 1 and 2 were too rich. He said it needed a new mass air flow sensor and so I replaced that. About 20 minutes after I drove it off the check engine light came back on so I brought it back in and they said the code came up for some type of emissions control and they spent over two hours checking it over and all they found was a small vacuum leak which he fixed. A couple days after that the check engine light came back on and the car was again running rough. It was idling and hesitating at stop lights and also making a hissing type noise when you accelerate. I brought it back in and they spent eight hours looking at it and could not find what was wrong. The same codes came on as before--being too rich. He thought it might be the spark plug wires even though the car only has 47,000 miles on it--he said a couple looked like they were old. He suggested replacing those and/or the spark plugs and see if that helps. He checked everything--the catalytic converter, the exhaust pipes, and can find nothing wrong. He wants me to bring it in Monday to see about the spark plug/wires. I am so frustrated with this car. Does anybody have any suggestions? Should I replace the spark plugs and/or wires? Would the dealer know more?

replace oxygen sensor even though light is not lit. should solve your problem.

If you are still getting fuel trim codes??? then they need to start with the basics.

Has the fuel filter been replaced, it controls fuel pressure & should be replaced every 60K miles OR 5 years?

The air mass sensor sensor?? was it a complete brand new BOSCH unit OR something cheap from ???? When replacing the air mass sensor was the rubber seals at the connecting adaptor replaced??

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I am a little skeptical that this would cause rough idle. I could be wrong of course...

Always trust MB Doc

It strikes me as bad wires but you should get a misfire code I would think.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

The article in the link below is for the W203 supercharged engine, however the symptoms very closely to what you describe. I just thought you may wan tot take a look at the article.

Site link:
https://www.mbwholesaleparts.com/StarTuned/html/index.html

Article Title & link: Engine Performance Complaint from "False Air" Model 203.040/740

https://www.mbwholesaleparts.com/StarTuned/pdfs/Mar08_ServiceBulletin.pdf