I finally broke down and took my car to the mercedes dealer and asked them to fix the Check Engine Light. I had built one of the code readers and it was telling me I needed a new speed sensor but I didn't want to go replacing parts willy-nilly.

The dealer said, "You needed a new O2 sensor."

That never came up on my LED tester. I said "Do you promise that will fix my CEL?"

He said, "yes."

"Go ahead," I replied.

I called this morning and the car is fixed. $300.17 but worth every penny. I had spent hours with the LED tester, clearing codes, trying to figure out what they meant and being sent off in all directions. The moral of this story: Not all things are best done yourself. Yes, I could have easily changed the O2 sensor, and I have done so on other cars. Yes, I could have bought the part for less than I paid the dealer. But the point is, how many other unnecessary parts would I have purchased and installed because I didn't know the real problem?