Suddenly the remote keys don't unlock the car

Is this with the key in the door or with the key fob remote?
Check the basics first. Is the battery in the remote working? If in doubt replace it with a good quality battery.
Next check the fuse. It should be the fuse box under the dashboard, possibly under the steering wheel. I think it's fuses 49, (10 amp-red), and 50, (25 amp-clear/white).
if these are OK it could be one or more of the servo units in the doors at fault.

Your keyless remote is equipped with 2 different systems. There is the RF system (long range, lock control only) and the IR system (short range, locks and convenience features such as remote window roll-up and roll-down). It sounds like your RF system isn't working. The only way to test the system, that I know of, is with a Mercedes scanner. It is likely a bad remote.

does your unlock button work in car? If it does, do you have another keyless entry key fob? If so does it work? If you just replaced the battery you can try removing battery and cleaning contacts within remote. I know some of the nissan altimas had the same issue and the only solution was a new key fob. Good Luck

I tried replacing battery in my remote. Sometimes it work and sometimes it doesn't. I think it could be a wiring problem in the remote. I know it's a major paun having to hurry up and put key in ignition befofr alarm starts going off.

Hello
If the keyholes were not used for some time - then the grease in there could turn into stiff mass. Try put some wd40 in the keyholes and try to turn around with a key - watch out so that the key wouldn't break and stay inside the hole. Good luck

It's probably the key fob battery. Why can't you sue your ignition key to unlock the door?

Take off the door handle cover and use the plastic key that is in the back of your remote

hi yes this can be done, here is how
make sure you have all keys ready to reprogram close all windows and doors insert key into drivers door lock within 5 seconds of putting key in door lock turn key to unlock position 3 times this puts the key into programming mode ( door led should be on ) press and hold lock button, within 5 seconds of holding lock button press unlock button 3 times ( keeping lock button pressed at same time) then release lock button that should be it...to make sure correct programming is achieved key led flashes 5 times, and door led flashes 6 time, if you have more than one key do same as above within 15 seconds of doing the first key..
phew..lol, well i hope that will work for you..please vote thanks

Programming
When
Key batteries completely discharged. Key batteries not replaced within 60 seconds. Key added or replaced. DO NOT press remote control buttons during battery replacement.
How
Unlock vehicle. Enter vehicle. Close driver's door. Switch ignition ON and OFF. Remove ignition key. Press and hold unlock button Fig. 1 [1]. Press lock button Fig. 1 [2] briefly 3 times within 10 seconds keeping unlock button [1] depressed. Release unlock button Fig. 1 [1]. Some models: LED Fig. 1 [3] flashes slowly for 10 seconds. Some models: Central locking system locks and unlocks rapidly to indicate correct programming. Some models: Repeat above procedure if central locking system does not work and LED does not flash or LED flashes continuously

To change batteries remove panel back of key then screws on plastic bar
Batteries are available from your local motor factor shop
You might not need new batteries try re programing as described above

Hi, it’s pretty difficult to lose a part of the circuit as you describe. So assuming that you have placed new fresh batteries correctly in the key – making sure that the polarity is correct! (easy to get wrong!) the key will need to be synchronised to the cars ignition / management system. You don’t say which model or year your Merc is, but most have a similar way of re – synchronising the Key to the car. It’s pretty easy and you lose nothing by trying it anyway.

Synchronising the locking system
If the vehicle cannot be locked or unlocked, first check the remote control unit batteries (see below). If the battery charge indicator lamp lights up, this means the system needs to be synchronised.
To do this, direct the remote control unit at the car and briefly press the Lock or Unlock button twice. Within approximately 30 seconds, put the key in the ignition lock and turn the key to position 2. The locking system should now be operative.

Battery Check
Press either the Lock or Unlock button and hold for longer than one second. The light on the key lights up briefly if the battery is OK. if it does not light up, replace the batteries immediately. Mercedes will charge a small fortune for these, so try elsewhere, they are pretty common and will cost substantially less.

If re – synchronising the Key fails to solve the problem, check that the Central locking fuse in the cars main fuse box is OK.

Hope this solves the problem – Let me know? Good luck!