It’s always annoying when the key fob for your car doesn’t work. Fortunately, there are several steps you can take to address this issue. This article will explore some steps you can take to solve the problem of a Mercedes Benz key fob not working to unlock doors or a trunk. Specifically, this article addresses issues with unlocking the Mercedes Benz CLK, E C CLS, A160, C-Class C220, C230, C300, CLS, and ML models.
The specific problems discussed in this article are:
1. Your Mercedes key fob doesn’t work after changing the battery.
2. Your Mercedes key fob has stopped working with no warning or explanation.
3. Your keyless remote or key fob won’t unlock doors.
4. The panic button and/or trunk unlock features don’t work.
Your problem is likely that the fob can start the car but will not allow you to unlock the vehicle’s doors or trunk. In other words, it will not allow you to use the fob as a keyless entry remote. Here are some common reasons for this issue:
1. The key is defective.
2. The All Activity Module (AAM) is not recognizing the key signal.
3. The AAM is not receiving signal at all because of an antenna issue.
So where do you begin to figure out the cause of your specific problem?
A good first step is to change the keyless remote battery. (If you’ve already done this, keep reading.)
When you press the button on your key, look at it carefully to see what happens. You should see a red LED light, and this should appear whenever the Lock, Unlock, Panic, or Trunk button is pressed. If you don’t see this light, the problem is that your key fob is defective.
If the key fob appears to be working, see if the remote is transmitting a signal. This signal will be infrared, which you can’t see with your eyes, but can detect using a cell phone. If you point your phone’s camera at the remote, you should be able to see a light when you press a button on the remote. Again, if you don’t see this light, your key fob is defective.
Next, try taking the emergency metal key out of the fob and using it to manually unlock the car’s doors. Then check the fuses for the central locking circuit (controls the vacuum operated door locking mechanism, which uses pressure to lock all the doors at the same time), antenna module, and AAM. Your Mercedes Benz receives the Lock or Unlock signal from antennas installed on the car, so a bad antenna module fuse can cause your key to be unable to unlock doors. Depending on the type of Mercedes Benz you have, the fuse will be located in one of the fuse boxes in the engine bay, on the side of the dashboard, under the backseat, or in the trunk. If you find a blown fuse, replace it (using the Ampere rating recommended by the manufacturer. Be careful not to use a higher rating as this can cause electrical problems.)
If none of these approaches have worked for you, it’s possible that your key needs some extra work to synchronize to your car. (Usually, this synchronization happens as soon as the key goes into the ignition, but in some cases, you will need to program the fob to synchronize to your car.) Here’s how to synchronize the key yourself:
1. Put the key in the ignition.
2. Quickly remove it within 10 seconds and hold down the Lock button.
3. While holding the Lock button, press Unlock five times.
4. Release these buttons and press the Trunk button once. This should synchronize your remote. You will be able to tell if it worked by pressing the Lock and Unlock buttons to see if they work.
Last, you may need to examine the AAM. (Unfortunately, if you haven’t resolved your car’s key fob issues by now, this step is a bit more involved and will cost some more money. Good luck.)
Start by reading the AAM fault codes with a diagnostic car scanner. You will have to purchase this (Foxwell, iCarsoft, and Bluedriver models are all recommended for Mercedes Benz), or you can ask a mechanic if you’re not comfortable doing this yourself. To use a diagnostic scanner, turn your car’s engine off and plug the scanner into the diagnostic link connector under the dashboard. Start the car and you should see some codes display on the scanner. Follow the manual’s code reading procedure to interpret these, or look them up online. If you determine that the module is defective or programming is necessary, the scanner will not be able to help you unless it is a more advanced variety (such as Autel Maxidas or SnapOn Solus). There are different levels of scanners from simple to complex. More expensive (and complex) ones can give you the same information your car’s computer is getting. (These higher end models probably aren’t necessary for diagnosing AAM issues, but if you enjoy working on your car they can be a great investment.) If the AAM is defective, you will usually need to have it reprogrammed by your Mercedes Benz dealer, or buy a new one. (You may be able to buy a used one for less money, but if you do this, make sure it has the same last four part number as your model.)
It’s always frustrating when your Mercedes Benz key fob doesn’t work to open the car’s doors or trunk. Hopefully, reading this article has left you better prepared to deal with this situation.