Hello,
I have C220 CDI of 2001 (CoupeSport), when I turn on my key the light of the heat plug come one after +/- 2 seconds he become off, that is ok, but after to start the engine the light come back on for +/- 50 seconds, I think is not normal, it was not like this before, I suppose I have one plug is broken, how I can determine which one is broken, or I need to change all, what is the best solution. Thank you for your help.
Regards,

The best thing is to change everyone :wink:

I believe each plug can be individually tested with an Ohm-meter. If the resistance isn't up to spec, that's the one needing replacement. At least I know this works with the w126, I believe it would work in your case too. Normally I would take Frode's advice in that usually once one goes the others aren't too far behind but since yours is new, I'd say you just happen to get a bad one, check the ohms and replace the bad one.

Good luck!