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Mercedes-Benz C-Class: Why is My Alarm Going Off?

A dead or low car battery can make your car alarm malfunction. Or, it could be a deeper issue with the two separate batteries that power the alarm's siren. Follow these easy instructions to fix the problem.

By Richard Bowen - May 19, 2015

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

There are a few common causes that will set your alarm off in your Mercedes-Benz. The most common is an issue with the battery. However, if you are unsure where to begin, continue reading as this article will cover the possible causes for an errant alarm.

DIY Cost Breakdown:

  • 12 volt battery: $270
  • 2 AA batteries: $1
  • Siren unit: $100

Materials Needed

  • Voltmeter
  • Tire iron
  • Car jack and jack stand
  • Sockets
  • Ratchet
  • Flat head screwdriver

Step 1 – Check the car's battery

A low or depleted car battery can lead to a malfunctioning car alarm. Test the battery, and make sure it is putting out 12 volts. If the battery reads below 12 volts, replace it and test to see if the alarm works properly.

Pro Tip

Even a battery that shows voltage less than 12.6 to 12.7 volts can cause problems.

If the issues persists and the battery can cleanly be ruled out, it's time to move on to Step 2.

Step 2 – Repair or replace the siren's batteries

The siren mated to the car alarm has its own batteries. Located under the driver's side fender, the siren sounds every time the alarm activates. If one or both of the batteries goes bad, they can leak and reek havoc on the siren's components. This damage can cause the alarm to malfunction. Fortunately, the repair is a relatively easy fix.

Figure 1. Alarm siren (front view).
  1. Put the car in park, and engage the emergency brake. If manual transmission, place the car in gear and engage the emergency brake.
  2. Jack up the car, and rest the car on a jack stand to ensure safety.
  3. Use the tire iron to remove the left-front wheel.
  4. Remove the pins that secure the left-front fender liner to the vehicle. Remove the liner.

  5. Reach inside, under the fender. With ratchet and socket, remove the three bolts that hold the bracket (and siren) to the car.

    • Figure 2. Inside of siren showing damage from corroded batteries.
    • Figure 3. Circuit board underneath siren's internal components.
    • Figure 4. Siren enclosure (cut in half), and internal component.
  6. Install new batteries and re-install the siren. Alternatively, install a new siren if the unit is too damaged to work with new batteries.
  7. Re-install fender liner and pins.
  8. Re-install the left-front wheel.
  9. Lower car, and test alarm system.

Pro Tip

Undo the fender liner pins by first lifting out the center post on the pin with your flat head screwdriver. The lower portion should then come out easily.

Step 3 is an optional step for Canadian-spec cars. Are you in Canada? This is for you.

Step 3 – Reset the alarm (Canada cars only)

Cars built for the Canadian market have a reset switch for the alarm system.

  1. Find a switch on the overhead console that shows a car being lifted from the rear.
  2. Remove your key from the ignition, and then press this switch.
  3. When the light comes on, the alarm system is deactivated.
  4. To re-activate the system, unlock the car.

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