I Have A 2002 ML320 W/96000 Miles. I've Only Had It For 2 Weeks. I Replaced The Battery 1 Week Ago And Then 2 Weeks To The Day The Check Engine Light Came On And It Started To Idle Rough. I Unplugged Battery And Reconnected The Light Went Off And It Ran Great. Later That Night Light Came On Again. Changed Spark Plugs (which Hadn't Been Changed In A Long While) And Added Fuel System Cleaner With Higher Octane Gas. Started To Get Better, Next Day Worse. Took To Autozone And Got Codes:p0303-misfire Cylinder 3, 90203-malfunction Exists In Electrical Circuit For Cylinder 3 Fuel Injector. What Should I Do Next Any Suggestions Please?!

either the injector is improperly connected to the wiring harness OR the injector is BAD..

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Thank You I Will Check EVERYTHING Tonight. I Have An Appt Tomorrow Am With My Local MB Dealer To See What They Have To Say About This Whole Mess. If The Injectors Are Bad Should I Replace That One Or All (stupid Question I Know) And Would It Be Easier To Have Them Do It Or Should I Do It Myself? (the Spark Plugs Were Fun To Do W/o The Right Tools). Also Will The Computer Reset Itself Once The Problem Is Fixed Or Do The Codes Have To Be Erased?

12/20 Okay so I took it to by MB dealer and they said it was the injector and that I could change the plugs since it was coming up on 100k miles. That makes me wonder b/c I just changed them-so they didn't even look under the hood?! They did charge me however to tell me all that. Needless to say I am going to order the injector and install it myself. (I'll save 250.00) My only question now is where is the 3rd cylinder? I don't have an owners manual and can't find anything that tells me the cylinder order. Please help me... thank you

See http://www.benzworld.org/forums/w163-m-class/1265523-firing-order.html?highlight=firing+order

Thank you so much azinn. That is exactly what I needed!!