Loose GP wire connection -- Ck. simple solution first
 

I got a glow plug light on after start up on my CDI during 6400 mile cross country trip. The car was in for a ruptured fuel line going to fuel filter which is another story. MB tech. put it on Star machine which indicated to check #4 circuit wire and controller.

I was advised to replace all glow plugs. This is standard MB practice which does make sense if you indeed have a bad glow plug. The thought process if all glow plugs have about same lifespan and this will reduce trips to service for busy owners.

I declined and said I'd wait to get home. As it is easy to remove engine cover, I decided to replace the faulty glow plug. When I got into top of engine, I decided to try each connector by pushing on them (I'm a noodling monkey....what can I say?). Sure enough #4 pushed down with a click. I asked my local mechanic who has worked on sprinters if I should go ahead and replace the plug. He told me he was fairly sure the connection was the problem. He said, "Clear the code and see if it stays cleared". I did that and that seems to have been the problem.

The glow plugs were replaced at 75k miles several years ago by the previous owner. Was the wire loose all that time and only became a poor contact when removing the engine cover and disturbing the wire? Was it loosened while the fuel line was worked on?

Moral: Ck. the simple things first. Listen to the diagnostic system. The Star system said, "ck. wire circuit". I assumed the MB Tech. knew that it was the glow plug and could skip the advise.

You did the right thing. Good job. :)