That is a very good question.. I had a similar problem with starting my vehicle.. In my case, I will turn the key and there was nothing going on. The lights on the dashboard did not even dim.. After several unsuccesful trips to the MB stealership, it turned out to be a loose wire in/around the starter.. I will ask the mechanic to clean and tighten up the connections to the starter and to check if the starter solenoid is working. It should make some noise when you turn the key to start the vehicle.. a click or something..
The mechanic should be able to test directly the starter, too, and see if it works or if current, and how much, is getting to the starter.....
Make sure the battery terminals are very clean. As odd as it can sound, a couple of weeks ago, I was helping a friend trying to find out why his vehicle will not start at all.. Everything tested ok, lights, relays, main relay, solenoids, battery load test, etc.. but there was no crank whatsoever.. We went to eat lunch and we decided, just for the heck of it, to thouroughly clean the battery terminals and clamps, which looked a little sulfated, and we left the battery charging..
When we came back, we put the battery and the car started right away and has not failed since... We still can't figure out what was wrong..
By the way, when your car fails to start, you don't get a "Start Error" message on your dashboard.. right..? There is a known problem with the DAS (Drive Authorization System) that causes that problem...
Thanks for all the replies and advice.
..... How can I tell if it is the starter that is the problem?