Those keys shouldn't need to be re-synched. It sounds like the key is no good. Try aiming the key at one of the Infra-Red receivers in the door handles at close range and see if that works. If it does, it's possible that only the RF (radio frequency) portion of the key has failed, or you may have an antenna issue. The fact that the car starts doesn't really mean anything. MB "smart-keys" will start the car even if the batteries are missing. Also, check the little LED when you press and hold any button. Does it blink once? If so, it's transmitting (or at least it thinks it is).