This happened on my E300 last year. Changing FOB batteries did not work so I had to tow it to the dealer, leave it for two days till the new FOB came in and $265 later all good. Seems like over kill for a new key but thats what you may have to do.

Good luck.

__________________
Treetops
06 E320 CDI 127K Miles
87 300TD 231K Miles

99 E300 269K Miles-Sold