'06 C230 error code P0718 stuck in limp mode!

What should I expect to hear and any guess at an approx cost? I've read that it could be a speed sensor a valve body or worse. I've also read that this is a fairly common problem but Mercedes refuses to acknolege any fault. Car has 112,000 miles on it and otherwise in very good shape.
Any chance that the dealer will cover anything?



