This is quite a simple task if you have changed the batteries or for whatever reason the key no longer correctly locks and unlocks the car as it used to.
There are principally three variations on a theme and these are outlined below:
The following instructions are for the square flip out key (not the later DAS pear shaped keys)
Model W124 (with code 885)
Model W124 model year 01-03-96
Model W129,W140,W202 model year 01-12-94 up to 31-05 97
Model W210 model year up to 28-02-97
Point the remote key towards the rear view mirror receiver.
Press the remote button just once.
NOTE: For vehicles model year up to 12-93 – lock or unlock car with the remote key flip out blade within 30 seconds in door lock
Mercedes C class models from Jan 1994 with control modules 202 820 40, 26 and 202 820 43 26. Turn on ignition to position 2 with folding key within next 30 seconds.
W210 E class cars as from December 1994 with part numbers:
210 820 21 26
210 820 22 26
210 820 27 26
210 820 28 26
210 820 32 26
Point the remote key at the interior mirror, press remote button twice then turn key in ignition lock to position 2 (within 30 seconds).
Test function of key, it should now be synchronised!
If the key fob now locks and unlocks the doors correctly, but the red and green lights flash alternately red/green unusually on the rear view mirror – replace the batteries in the key fob remote. This alternate flashing is the Mercedes indication of low batteries in the remote fob/flip key – it is not widely publicised.
You can watch a video of how to change the key fob batteries here.