If your vehicle is not recognizing the key and displays a message such as "Key not recognized," you most likely are using the wrong key or have a dead key fob.
Other possible problems include the key is damaged (even if it unlocks the doors), a bad All Activity Module (AAM), or a faulty Electronic Ignition Switch (EIS).
Start by making sure that you have the correct key. If you have a keyless start option, remove the START button from the ignition and insert the keyfob in the ignition or press the START button with the key to start the engine.
You may have to hold the key next to the steering column on some models while you press START.
If you have a spare key, try starting the car with that key.
Here are a few things you should try if you are getting this error message.
If you dropped the key in water or washed it with your laundry, the key may stop working.
In some cases, allowing the key to dry for a few hours will let you start the engine. Remove the battery (and the circuit board when possible) from the key and allow the key internals to dry.
If the key was dropped to the ground, it might stop working as well. When the key is dropped, the small coil inside the key can get damaged. A Mercedes-Benz key typically doesn't fail with a single drop but drops it several times, and you may end up with a key that won't start or unlock the car.
If you remove the circuit board from inside the keyfob, you should see a small coil. The coil is soldered at both ends. Inspect both ends and make sure they are still connected to the circuit board.
Try Spare Key
If you have a spare key, try starting the engine with the second key to see if it works.
If the second key starts the car, we know we have a bad key. A bad Mercedes-Benz key may be able to unlock doors but not start the engine. That's because a Mercedes-Benz key sends two different signals. One for opening doors and the other RFID chip used to authorize vehicle start. While it is unlikely to happen, it is still possible that the RFID chip integrated into the key is bad.
If the spare key does not work, the problem is not the keys but the car.
Supply Power to Battery
The main car battery may be partially discharged. When this happens, a number of low voltage fault codes get stored in the various control modules. More importantly, modules such as the Electronic Ignition System (EIS) may not have enough power to power up correctly and recognize the key.
This scenario can be challenging to diagnose because when the car battery is practically discharged, the car lights and instrument cluster still power up.
What you can do here is connect a jump box (or jumper cables) to jump-start the car. Press unlock on the keyfob and try to start the engine. In some cases, jump-starting will allow the vehicle to recognize the key and enable you to start the engine.
Disconnect the negative battery terminal to reset the ECU. Resetting the ECU will sometimes allow the car to recognize the key.
You may lose a few memory settings, such as the window auto-close feature and power seat memory, but those are very easy to set on your own.
Next, check all the fuses to make sure there are no blown fuses in the car.
Lastly, make sure to replace the key fob battery. It can be why your Mercedes-Benz does not recognize the key, especially on models that have a keyless start. When you remove the START button and insert the key in the ignition, you should be able to start the engine even if the key fob battery is dead.
The tips in this guide work in most cases. If none of these tips helped, your vehicle will need to be diagnosed with a professional scanner. Mercedes-Benz Star Diagnostic scanner is the best option to diagnose the Mercedes vehicles.
If you are looking to diagnose other makes and models, Autel MAXICOM and Launch All System Scanner are good options. With any of these scanners, you should perform an all-system scan to determine what fault codes are stored in the various modules.