Just today my '00 300M started running rough. Most noticable at idle\n- like a cylinder not firing.\n\nEngine light came on.\n\nAfter parking it for a while, and starting it, it idles rough, but no\nengine light. Engine light only comes on if I drive it a little.\n\nCodes displayed on odometer are P1684 and P0302.\n\nThe 1684 is a battery disconnect code (it has the original battery,\nwhich hasn't been disconnected for years - if ever, so perhaps this is\nan indication of a weak battery - although it turns the engine over\njust fine when cold). The 1684 code is said to not be responsible for\nthe check-engine light.\n\nThe P0302 code (if I'm right) indicates misfire in cylinder 2. This\nwebsite:\n\n[URL]http://www.aa1car.com/library/copign.htm[/URL]\n\nsays that I should also expect to see either a coil code that\ncorresponds to the same cylinder (P0351 to P0358) or a code that\nindicates an open or shorted injector in that cylinder (P0201 to\nP0208). I don't see any of those codes. Just 1684 and 0302.\n\nMight this set of conditions represent just a bad plug then? (orginal\nplugs).\n\nCar is exactly 8 years old tommorrow - bought it new back on Nov 1,\n1999.\n\nComments?\n\nAnswers?\n\nRecommendations for aftermarket plug brand (I might as well start\nthere) ?\n\nHave never taken off a coil pack before - any special tools, or is it\nself-evident?\n\nWhich one is cylinder 2?