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Car Alarm Horn Goes Off Randomly

Is your car alarm going off for no reason? This article reviews the common issues that can cause a car alarm to go off randomly.

Mercedes’s alarm may go off even when no one tries to steal the car. We have seen many cases when a Mercedes alarm (siren) goes off during the night while the car is parked when you try to lock the car or during extremely cold weather.

This past week, we worked on a Mercedes-Benz where the alarm would go off randomly in the middle of the night. This Mercedes-Benz would be parked for hours, and then the car alarm would go off for no apparent reason. No one was trying to get into the car or was close to the car.

This guide will teach you how we fixed a Mercedes alarm going off. Also, other common issues can cause a car alarm to go on and off.


First, let’s take a look at what many Mercedes owners experience:

  • Mercedes alarm goes off randomly
  • The siren goes off and then kills the battery
  • The horn randomly goes off
  • The alarm keeps going off, and no one is by the car
  • The factory alarm goes off randomly

Mercedes’s alarm going off randomly doesn’t affect only one model. These symptoms can affect several models, including A-Class, E-Class, C-Class, SL-Class, CLK, CLS, GLA, GLS, GLK, GLE, ML, GL, R-Class, and quite often the S-Class.

How to stop the Mercedes alarm

  • Press the unlock button on your key fob

Common Causes

Let’s look at some of the most common problems that trigger a car alarm randomly.

Even though these problems apply to Mercedes-Benz cars, a few apply to other makes.


Car battery triggering alarmA weak battery is the most common cause of a car alarm going off randomly. If your car alarm goes off in the middle of the night, an old battery is often the problem.

If your car battery is over five years old, it may no longer hold a proper charge.  If the battery voltage drops below 11.5 volts, your car alarm can go off (when programmed at the factory).

This happens when the car is parked for several hours, typically overnight or when the weather gets extremely cold, which decreases battery performance.

Why does a car alarm go off when the battery charge is too low?

To prevent vehicle theft.

A thief will try to discharge the battery to disable the alarm and other safety features. Car manufacturers program the alarm system to go off if they notice the battery starting to discharge or drop below a specific voltage.

This is a common problem in many luxury cars, including Audi, Lexus, BMW, and Acura.

We recently had an ML / R – Class where the car alarm would go off during the night. What complicates this case, even more is that the car would start without a jump start.

After further troubleshooting, we noticed the engine slowly turned over and struggled slightly. This is an indication that the car battery is not performing well.

Parasitic Draw

speaker draining car triggering car batteryA parasitic draw can also cause your car alarm to go off randomly. A parasitic draw happens when you leave lights on or have a system such as a radio or an amplifier that drains the battery when the car is parked. For example, if you leave an accessory powered in the cigarette outlet, amps are always on, or a module doesn’t go to sleep, it will start to drain the car battery.

Again, once the battery voltage goes under 11 or 11.5 volts, it will cause the car alarm to go off. This problem can not be fixed by replacing the battery.

Anti Theft Siren

mercedes sirenThe battery in your alarm siren can go bad, causing the alarm to go off. 

You lock the car doors, and a few seconds later, the car siren goes off for no apparent reason. Hazard lights start to flash. Or, in the worst-case scenario, the alarm goes off in the middle of the night.

Many cars, including Mercedes-Benz, have tires with built-in batteries inside them. When the siren batteries fail, it can cause the alarm to go off.  Change the siren batteries or replace the siren itself if found defective.  In most cases, the siren is near the driver-side wheel well. Check out this post on MBWorld to learn about a few cases with this exact problem.

Newer Mercedes-Benz cars are equipped with both a Horn and a Siren. The horn is in front of the radiator. At the same time, the siren is in the engine bay or under the driver’s side wheel well.

Steering Wheel / Clock Spring

my car alarm went off in the middle of the night car alarm randomly going off at nightIf the horn button is stuck pressed or the clock spring is damaged, it can cause the horn to go off. But this problem will not happen only when your car is parked; it will happen at any time, even when driving.

Door / Trunk Hinges (Latch)

car alarm keeps going off call policeA door or trunk hinge could cause the alarm to go off. Sometimes, the hinges are not aligned properly.  Car parts contract and expand the most during extreme cold and hot weather. This can trigger the car alarm to go off.

SAM Module

Your SAM module could be the culprit as well. The rear SAM control unit, typically in the trunk, could fail if water gets inside. Control Unit N10/2 is what controls Automatic Theft Alarm (ATA), often referred to as Siren.

Motion Sensor / Tilt Sensor

Any motion sensors mounted inside the vehicle can malfunction, triggering the alarm system to go off randomly.  Motion and tilt sensors can detect if someone breaks into the car but also detect vehicle movement. If they fail, the car may think someone has broken into the car or is being towed away without authorization.

The alarm module receives inputs from several sensors and micro-switches. If a faulty sensor is present, the car may think a theft is in progress, triggering the alarm and flashing the turn signal lights.

Further Troubleshooting

It can be difficult to diagnose a car alarm that goes off randomly. First, start by checking the battery age. You may consider replacing it if it is over 5 or 6 years old. You don’t want to wait until you get stranded.

If replacing the battery doesn’t fix the problem, the next step is to read the fault codes using an OBD-II scanner that can read Mercedes-Benz fault codes.

If your hazard lights are flashing, but the siren is not making any sound, it is possible that the wires to the siren have been cut or the siren is disconnected.

What Can I Use to Troubleshoot Mercedes-Benz Problems/Issues?

If you’re experiencing problems with your Mercedes-Benz, the YOUCANIC full system diagnostic scanner can provide invaluable assistance in troubleshooting the issues. By connecting to your car’s onboard computer, the scanner can read fault codes and provide detailed information about any problems with your engine, transmission, brakes, and other systems. Additionally, the scanner can perform diagnostic tests on various systems, such as the ABS, airbag, and steering systems, to help identify the underlying cause of any issues.

In addition to providing diagnostic information, the YOUCANIC scanner can save you time and money on repairs by enabling you to perform simple fixes yourself. For example, if you notice the check engine light on, the scanner can assist you in identifying the problem and determining if it’s a simple issue you can address at home or if you need to take it to a mechanic. This can help you avoid unnecessary trips to the repair shop, saving you both time and money.

Overall, the YOUCANIC full system diagnostic scanner is a powerful tool to help you maintain and troubleshoot your Mercedes-Benz. It’s an excellent investment for both DIY mechanics and professional technicians, as it can assist in diagnosing and resolving even the most complex automotive problems easily, saving you time and money on costly repairs.

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