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Mercedes-Benz C-Class: Why is My Key Fob Not Working?

You rely on your key fob to unlock your car's doors as well as to get it started. There are several reasons why your key fob might not work. Continue reading to find out those reasons.

By Kate Halse - May 14, 2015
Contributors: blackcls63

This article applies to the Mercedes-Benz C-Class w204 (2007-2014).

You go to start your car one day, but you get no response from your vehicle. Your first thought might be a dead car battery, which can sometimes cause the key fob to stop working. Before you call the tow truck, however, it's important to consider other reasons why your key fob might not be working. The solution could be as simple as swapping out the key battery with a new one. Here's a look at some common reasons why your w204's key fob isn't working.

Material Needed

  • Replacement key battery

Step 1 – Check the key battery

Some owners report an error message, such as "key not detected," or limited range when the key batteries are dead or very low. Fortunately, changing the key battery is simple and can be accomplished by using the valet key that comes included with the fob. This process generally takes less than a minute to complete.

Figure 1. Replace the key battery.

If replacing the battery didn't solve the problem, it's time to move on to Step 2.

(Related Article: How to Replace SmartKey Battery - MBWorld.org)

Step 2 – Ignition activated but car won't start

The Mercedes-Benz keyless Go card can cause a no-start situation if the signal is weak or blocked. Many owners with this problem report that the ignition light will illuminate, but the car won't start. In order for it to fully transmit the signal, the card needs to be in close proximity like in the front pocket of a shirt, and not in a laptop bag.

Figure 2. Check the range of the keyless Go keys.

If the keyless Go seems to be functioning but the key still isn't, it's time to try Step 3.

Step 3 – Both key fobs have stopped working

If neither of your key fobs unlocks or starts the car, there's a good chance that the issue is the car's battery. Some owners report that their cars failed to start even after a jump start, but a later trip to the dealership indicated a dead car battery. Since a trip to the dealership is often costly, it's a good idea to try jump starting your car before calling for a tow to the dealership.

Figure 3. Jump start the car's battery.

If the battery has sufficient voltage but the key still isn't working, it's time to try Step 4.

(Related Article: How to Jump Start Your Car - MBWorld.org)

Step 4 – SmartKey doesn't work

If your SmartKey isn't working, you might need to re-sync it. The main symptoms are the key won't unlock the doors or start the car. You may see a blinking red light when you press a button.

  • Insert the key and turn it to the "ON" position, then press and release the lock button on the key.
  • Press and hold the remote's unlock button.
  • In 1-2 seconds, press the remote's lock button five times in a row while holding the unlock button.
Figure 4. Re-sync the SmartKey.

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