Mercedes "Key Not Detected" Message Mystery Explained

In the keyless world, seeing the "Key Not Detected" message in your car is pretty scary. If it's a new Mercedes, it's even scarier than usual because you literally don't know what to do and if there is any backup version of starting your car when the fob is dead or not detected for any other reason.

Key not detected message highlights

• Common reasons:key fob battery dead, 12V battery problems, antenna issues
• How to fix:check and replace the batteries, try another key fob
• Possible consequences:car won't start
• Priority level:High
• Can you drive?No
• DIY repair:Possible
• Repair price range:$5-$300

Reasons for this key fob error message

Whenever you see this message on the dash, it means that your Mercedes can't recognize the key properly. It means that you will not start the car just by pushing the button. Now, in different generations, the reasons and also the ways to solve the problem may be different. I personally had experience with 2015 and 2023 models of Mercedes-Benz cars in terms of dead fobs. And I will tell you about both cases.

But first, let's look at the possible reasons why this may happen.

1) The key fob battery is dead

One of the most obvious and common reasons for this situation is the dead battery. In modern keyless-system fobs, batteries usually don't live for ages. They are able to power the chip for 10 to 12 months even if the fob isn't used much. This explains why your spare fob may not work, too.

So, the obvious fix is to replace the battery in the key. How you do it depends on the generation.

In 2015, Mercedes had fobs that you could open using the metal key. Take the metal key out, turn it 180 degrees, and insert it back into the small cavity on the same side that you've taken it from. Press it a little and the fob will open letting you get access to the key. It was the CR2025 battery there.

In new Mercedes vehicles, you will need to take the metal key out, then keep holding the latch open and slide the plastic cover of the key fob. Now, you have a special slot that you can take out with your nail or a small screwdriver. The battery here is CR2032.

2) The software glitch

In older Mercedes vehicles, there was a pretty common glitch that could throw the "Key Not Detected" error. If you just press several times buttons on your fob, it should fix the problem temporarily. But to deal with this issue, you need to go to a Mercedes-Benz dealership and ask them for a software update that will fix this communication error.

The software update was issued in 2016, as far as I'm concerned, and it fixes the communication error in cars earlier than that model year.

3) Key glitch

It's not uncommon for keyless technologies in cars to glitch sometimes. If your key is dead but the battery seems to be OK, you can try one simple trick. Take the battery out of the fob, let it sit for several seconds, and put the battery back. For so many Mercedes owners this cheap trick was the helpful way to get rid of the "Key Not Detected" issue!

Try this before you go to the dealership and maybe this will save you several hundred dollars.

4) The car battery is dead

While the low voltage in your car battery may trigger a lot of other error messages and problems, the "Key Not Detected" message is still one of them. If the battery is dangerously low, the voltage will not be sufficient for starting the car. But it won't also be good for electronic modules and "brains" of your car that don't have the possibility to boot properly.

So, charging or boosting your car battery may help in this situation. If the 12V battery is older than 4 or 5 years when it starts malfunctioning, it's better to just replace it with a new one.

Getting to your car when the key fob is dead

Mercedes has several different ways to start your vehicle and to get into it when the fob doesn't respond. At first, you had cars with visible keyholes, then they started hiding the keyholes under plastic covers. And in the recent generation of vehicles, they hid it even further.

Let's see what you should do:

1) Older generations with visible keyhole

Getting into the car is easy. You should take the metal key out of your fob by using the little black button on one of the sides of the fob. Then open the door with this metal key and insert it back into the fob.

After that, use your nails to hook the engine start button and get it out of its slot. There, you will see the slot for the narrow part of your key fob. Insert the fob there and turn it just as you would turn a manual key in the ignition cylinder of the older car.

2) Generations with hidden keyholes

Later on, keyholes became outdated and car manufacturers started hiding them. If you don't see the keyhole on the door handle of your Mercedes, you will need some more action. After taking out the metal key, look down the handle. On its right part which is the small metal cover, you will see a little cavity. Insert the metal key there and push it up like you are pressing a release button.

Now, the plastic cover is in your hands. You see the keyhole. Insert the metal key into the keyhole and open the door. Then, you may have to do everything like described in the previous part or you will need to find the designated area to place your key fob. For this, you better take the driver's manual and find where this is in your vehicle.

3) Newest Mercedes cars with hidden door handles

If the door handles in your Mercedes car are hidden and the fob is dead, you will need to press to one side of the handle (closer to the wing mirror) and then just use your fingers to hook the door handle and take it fully out. When you do this, you will see a small keyhole under the handle.

Take the metal key out of the fob and open the door manually. Now, you should open a little box located in the bottom part of your central console. In this box, you will see a rubber mat and the key pictogram. Locate your key fob directly onto this pictogram. Now, you can start your Mercedes car just by pressing the engine-start button.

When will you need professional help?

Usually, you can handle this on your own. At least, you can temporarily fix the problem and drive to the dealer without paying for a tow truck. By this, you will already save several hundred dollars. But unfortunately, there are situations that don't allow you to deal with the problem without professional tools and expertise.

Usually, you will need help in such situations:

• Your Mercedes needs software update or reprogramming. If the "Key Not Detected" message pops up really often and randomly, this may be because of a software glitch that is common for 2012-2015 Mercedes cars. This may be fixed by software updated in the dealership.
• Nothing works and the car won't start. If you tried every method described in this article and the message is still there on the dash and your car won't start, then you will probably need some professional assistance.
• There are other warning lights and messages. Almost any other messages may mean that there is a more serious problem and the "Key Not Detected" message is just one side of it. A proper inspection is urgently needed.
• The Mercedes repeatedly throws this message. If you changed the battery in the fob and the message is still shown, it means there is something else that triggers the issue.

Whenever you see this message, now you know what to do. In most cases, you will be able to solve the problem on your own without paying a dollar to the dealership. Also, now you know every reason why this can happen to your vehicle.

Final thoughts

Although seeing the "Key Not Detected" message in the Mercedes is not something you would like to experience, this is certainly not the worst problem with your car. You can still start the vehicle using the special procedure that Mercedes though out for these cases.

Also, you may want to take the driver's manual for your model of Mercedes and see how exactly it's done for the car. In different generations, the manual start option may have various aspects.