A good start is to visit JimF's website at:
http://pages.prodigy.net/jforgione/MB_S500.html
Check his item #6 on the list. Also great site for many troubleshooting techniques etc. Jim is an active member of this board as well. I first dealt with him after reading about his Cool Harness and found out this thing works great for my car (95 E420) as this engine tends to run hotter than I’d like it to especially down here in Texas. You may consider getting one for your car. I don't mind giving Jim an endorsement since I am using his product and very satisfied with it.
Make sure you read up on the proper way to read the LED flashes when reading DTC codes. You only need to press the LED for ~4 seconds then let go and then start counting. Once you finish the count, do the same again to make sure you only have same number of LED counts (thus same error). Write it down. To erase the code, you need to press and hold the button for 8-9 second AFTER it counts the error flashes as described above. After clearing the code, you should get only 1 flash when attempting to read the LED flashes. Again, refer to Jim's site for more details and he also has a list of all the error codes that should also match your car model.