Battery is good but car does not turn over in on position.

Looks the RFDI chip in the key is defect. Your Mercedes car dealer should be able to help you with that.
Fist to make sure it is a chip problem in the key and second by replacing the chip and the code and / or replacing the battery it an empty battery is the issue.

auto or manual ?

CHECK ALTENATER

Did you check your battery???
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sbb key programmer

Buy yourself a volt meter and check the batter voltage before and after it starts. You should read 12 volts before starting and around 14 volts once it starts and the alternator is charging the battery. Also check for loose connections between the alternator and battery. Check that the ground wire from the battery is secured to the vehicles frame. Good luck!

On Mercedes, and most German cars for that matter, you can only turn the key to the start position once. Then you have to turn the key completely off to turn it back to the start position again.

On almost all cars the steering wheel can also prevent you from turning the key, if the wheel is jammed to one side or the other. If you giggle the steering wheel left and right while turning the key, you can usually unjam it.

Hope this helps. Good luck and have a nice evening. Peter

model: 2001 C240 problem: wiper blades "park" almost in the middle of a windshiled when turned off. When wiper blades are on they cover complete range of a windshiled.

Please see my tip at http://www.fixya.com/cars/r5821173-doesnt_car_start_gasoline_engines . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

I have a 2001 C240 with the same issue. I removed the positive battery cable from the battery and kept it off for 30 seconds then replaced it. Everything was reset and the key worked.

possible nut holding mech. is slightly loose or stripped.