SLK230 coolant temp sensor repeat failure
 

I have had check engine light come on twice, both times they replaced the coolant sensor, the light is on again? Is this known problem for others? Is there hope for a real fix?

This is a long shot but, take a drop the positive lead of your DVOM in the reserve tank, ground the other and set you range to 20V. If you get a reading over .9 v, you really need to change your coolant, its acting like a rude battery. (seen this drive ford and toyota sensors crazy, for some reason alloy and acid in a glychol bath will do it)

Also make sure to pin out the harness, while I've not seen a SLK 111 harness failure it could happen.

Joe