Mercedes S Klasse diesel preheating issue

That sounds familiar. I had a similar issue with my old Renault. The glow plug light, which is what Renault calls the preheating lamp, stayed on, and it was slow to start. The check engine light also appeared later. Do you know which error codes are being reported? It could also be a car electronics issue. Have you inspected the preheating unit relay?

The error codes indicate a defective preheating control unit. The relay seems fine, but I will double check it. Where was the fault located in your Renault?

So, the error codes point to the preheating control unit. In my Renault's case, it was a corroded connector on a glow plug. I am no expert, but I would recommend visiting a workshop to eliminate more complex problems. Electrical faults can be challenging.

It turned out to be a defective cable connection to the preheating control unit. A new plug fixed everything. The total cost was 95€.