'05 clk500 with esp malfunction
 

The malfunction warning was coming on from time to time and now stays on. Everything else works fine. What do i need to check for in attempting to diagnose and repair myself?

Assuming that you have had no other alerts or symptoms of any kind and that no liquids have spilled or dripped in the console, then I would bite the bullet and pay to get it on SDS. Lots of indies have that so it shouldn't be awful cost-wise and there aren't any particularized "usual suspects" for this.

There are so many facets to the system that it'd nearly be blind luck that you'd be able to diagnose it without plugging in; even C3PO needed R2D2's help sometimes. ;)

Good luck.