I hope that you have better luck with a new smart key than I did. I purchased a new smart key for my 99 e240 (went through all the necessary requirements etc.) A very nice Key , but it would fail on starting, at random,(ie engine fired up then immediately died),on the second or third attempt, engine will keep running. So made complaint, after a day of testing I was issued with a replacement Smart Key, its better, but occassionally still gives the same problem, although engine will now keep running on the second start.
My old key I repaired , its OK, no problems.

That new Smart Key, took four attempts to get the engine going recently ( it just kept firing up the motor, then it shuts down immediately)
So I have made a second complaint to dealer (supplier of said key). He said it sounds like there is a fuel pump problem. But just use my old key, for now to see if I have the same problem.
Well to date there have been no problems with the old key