Age, wear and tear can take its toll on even the finest of cars. This even includes the Mercedes-Benz automobile. Regular maintenance can prevent some problems, but servicing your car as soon as a problem surfaces can keep things from escalating. Several types of door lock problems have been identified across the Mercedes line, but with identifiable solutions--including some you can handle yourself.
If all of the locks on your Mercedes fail to open (or close) first check your key fob to make certain that its battery is working. Replace the worn out battery, if necessary. If key fob is working, you may have a faulty key fob, which can be repaired or replaced by taking it to your dealer.
If just one lock won't open on your Mercedes, then the problem may rest with broken wiring, a bad solenoid, or perhaps with a blown fuse. Check your fuse box first to determine if a blown fuse is keeping the lock from working. If so, replace the fuse.
You likely have a shorted door lock actuator, which you can replace with a new unit for the lock that is not working. If a door unlocks, but will not lock again, then replace the door switch.
Perhaps your Mercedes locks work, but they open and close slowly. Apply WD-40 or equivalent product directly into the lock; open and close lock to recalibrate.
The National Highway Transportation and Safety Administration (NHTSA) issued a recall for select 2009 Mercedes S-class sedans. That recall involved fixing the side door locks on affected models which might have been exchanged during production. According to the NHTSA, "...the exchange of a front and a rear passenger side door lock would result in a passenger side door that cannot be opened by merely pulling on the interior handle and a rear door lock that can be opened with a simple pull of the interior door handle even if locked." If you own this make/model/year car, contact Mercedes-Benz at (800) 367-6372 for assistance.