GLK 2nd smart key not working
 

I recently purchased a 2013 GLK. I was given two keys with the car. I went to
use the 2nd key the other day and it would lock and unlock the car, open the rear door, but the car would not start. Is there a way to pair the key with the car again?

SmartKey oddity
 

I just joined your group but haven't figured out how to post a question so I'm trying it this way.
I own a 2014 E350 and my SmartKey has been acting odd. The car won't recognize the SmartKey at times and won't start the car, lock or unlock the doors. At other times the locking/unlocking functions work but it still won't start the car. I dismantled both my and my wife's SmartKeys, measured the battery voltages at 5.6V as both batteries were changed 4-6 months. My wife's SmartKey behaves correctly . . . any clues as to what may be goin' on? Thanx.

Hello and welcome to the forum! There are several reasons that the SmartKey may be acting strangely - I'm attaching a link below where they are outlined. Hope this helps you a bit.

SmartKey Issues