Door lock problem
 

My driver door lock will not actually lock the door. The central locking system locks the other doors and the door lock knob goes into the lock position, but you can open the door right up. When I open the door from the inside, the central locking system ulocks all the other doors properly. The key locks and unlocks the door lock.
Any ideas on whats wrong? Thanks in advance.

Re: broken driver's door lock
 

Had this exact problem on my car 500E a few months ago (105K miles). My dealer repaired it for a reasonable fee. He said it was the actuator mechanism (not a vacuum fault). It was under $200. I wouldn't be surprised, if you are inclined to DIY, to find that it is a repairable part that broke.

Graphite a no-no?
 

I have it on good authority from much more experienced MB owners than yours truly that graphite is NOT a good choice for MB door locks since it is electrically conductive and HAS caused failures of the alarm system, especially false alarms. I don't have any first-hand experience with this, however.