I have inspected the plug holders and they seem fine. I appreciate your help but I do not have the OBD reader here, so I need to figure it out. It seems like a simple question - but I have the Haynes manual and a MB Service manual which have not helped. Actually the MB Service manual indicates that the ignition coil # 2 is for cylinders 2 and 3 - so unless someone else can confirm tonight I will replace the first coil from the front of the engine, which implies cyclinders 2 and 3.