How to reset sercurity on 2002 c240. key wont turn. battery is full and new. car key has new batteries.

This is a Passlock system. See this post on how it works, what goes wrong and how to do a reset

http://ricksfreeautorepairadvice.com/passlock-fixes/

If the security light is blinking it will not start. The security system needs to be reset. You will have to have a fully charged battery to reset the system. With a full charged battery turn the ignition forward until the warning lights come on and leave the key in the on position for 15min. Turn key off and wait 10sec. then try and start.You may have to perform this procedure a couple of times, however, it is usually successful the first try. The reduced power light can be set by a low voltage/poor charge battery

With your remote unlock the door if you dont have a remote with your door key unlock the passager door and the rear hatch

Make sure its In park . Recharge battery first

Did you check your battery???
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sbb key programmer

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.

have you tried replacing the battery?

merc 220 wont turnover.new battery ignition lights come on then turn key nothing happens?

If you have noticed any strange electrical problems beside the alarm. Test a theory for me
before you start the car use the key in the drivers door lock lock then unlock the door hold the key in the unlock position till your cars window and sunroof open. Next Start the car close windows and roof turn on your AC high beam headlights fo lamps radio and wipers if your car runs little rough replace the battery and have the alternator tested. The alarm system is the clue happens when the alternator and or battery are not working properly PLEASE LET ME KNOW ON THIS

sounds like it needs a new steering lock