Car Alarm keeps going off reasons and fix

While car alarms are undeniably crucial in theft protection, they can become an enormous pain when they start going off randomly.

If your alarm goes off randomly, it’s either an electrical problem, a broken sensor, or a microswitch. Thankfully, the fix can often be simple and cheap for this type of problem, though specific issues may be harder to diagnose precisely.

We’ll go over the most common reasons for an alarm going off (other than actual theft) and ways to disable the alarm when you just can’t get it to shut up.

Why Does Your Car Alarm Go Off Randomly?

Anytime a component of the car alarm fails or has a fault, the alarm can go off. This could be a problem with the car battery, the control module, and the various sensors and micro switches. Even something as simple as a bad hood or the luggage latch sensor could cause the car alarm to be falsely activated.

This also means that damaged or shorted wiring can mean that the sensors and microswitches are not sending the correct message to the module, and this can, in turn, activate the alarm. Another culprit can be a poor or dying battery, which causes a low voltage that can mess with the alarm, among other car subsystems.

1. Bad Battery Voltage or a Dying Battery

Bad Battery Voltage or a Dying Battery

A weak or dead battery can trigger the car alarm, which is one of the more common reasons a car alarm goes off when it shouldn’t. This is the car alarm control module’s way of letting you know there’s a problem and that you must fix it before the system fails.

This might be because the battery may simply be too old and has weakened over time, causing low voltage. It might also be corrosion on the battery terminal that’s preventing the proper charge from getting to the alarm control module.

Another reason for low voltage can be if the car has been gathering dust for some time. The car recharges its battery while its engine is running, so the battery will drain if you haven’t let it run for several weeks or months. This effect impacts older cars more so than new ones.

You will need a voltage meter or multimeter to check your battery’s voltage. Please set it to the appropriate mode and measure the voltage on the battery terminals. If the reading is less than 10 volts, it’s likely your battery is faulty or empty.

Also, check if your battery terminals are damaged or corroded and whether the connectors are tightly fastened to the terminals.

2. A Faulty or Over Sensitive Shock Sensor

A Faulty or Overly Sensitive Shock Sensor

The shock sensor is usually found within the alarm control unit, detecting whether the car is experiencing some impact while parked and locked. For example, if someone is trying to break a window to get in.

However, a faulty shock sensor or one that’s set too sensitive can mistake the slightest nudge of something like an acorn falling on the hood, if a children’s playing ball escapes the park and hits your car, or if a cat climbs onto the hood and incorrectly triggers the alarm. The sensor mistakes this weak impact as someone tries to break a window or force their way into the car.

Some shock sensors are set to be very sensitive from the factory. This depends on the car make and model, whether the car came with an alarm, or which alarm was installed later.

Thankfully, the sensitivity of most shock sensors is adjustable, and sometimes, you can set it up through a diagnostic tool. If the sensor is broken, you’ll likely have to replace the whole alarm control module if it’s located there.

3. A Broken or Sensitive Ultrasonic Sensor

A Broken or Sensitive Ultrasonic Sensor

The ultrasonic sensor is another important one that will notice if something unusual is happening with your car, will notify the car alarm, and the loud whistle will scare away potential thieves. The ultrasonic sensor consists of two parts: a transmitter and a receiver. You will typically find them in the upper corners behind the windshield as two small cylinders.

An overly sensitive or faulty ultrasonic sensor can accidentally mistake outside ambient noises as someone trying to get into the car, activating the alarm. The ultrasonic sensors can detect sounds above what human ears can hear at some frequencies. This might make it seem like the car alarm is going off randomly, even though you couldn’t hear the sound that set the sensor off.

However, a sensitive or faulty sensor can be activated without threat. You’ll need a specialized tool to set the alarm unless it’s an Original Equipment Manufacturer (OEM) part. You can likely set it up with an appropriate diagnostic tool.

4. Opened Window or Other Outside Disturbances

Opened Window or Other Outside Disturbances

Sometimes, a slightly open car window can randomly trigger the alarm. Sometimes, it takes a strong gust of wind or an insect hitting the window to trigger one of the sensors.

Completely closing all windows and doors is always a good idea when leaving the car. Not only because someone can snatch items you keep in your vehicle but also because of false alarms.

5. Improperly Latched Door, Hood, or a Faulty Microswitch

Improperly Latched Door, Hood, or a Faulty Microswitch

If the hood or one door isn’t completely latched, it can trigger a latch sensor, causing the alarm to go off randomly. A micro switch in each door, the hood, and other movable panels actively read whether the door is open or closed. If the alarm control module detects that the door or hood isn’t latched to complete the connection, it will accidentally turn the alarm on, thinking someone is breaking in.

If this microswitch dies, it may falsely signal that the door/hood/cargo door is open when it physically isn’t. Sometimes, the dashboard will show that one of these is open, and the alarm will signal it with a specific tone. Other times, this may trigger the alarm fully.

Sometimes, an obstacle prevents the door from latching correctly and/or activating the microswitch. This may be dirt, debris like leaves, or damage to the latch from forceful closing or impact. Once this is clear, the alarm should work normally, and the dashboard and multimedia should show no open doors.

6. Faulty Wiring to the Sensors or Microswitches

Faulty Wiring to the Sensors or Microswitches

Even if all the microswitches and sensors are fine, a damaged, loose, or short-circuited wire running to one of the sensors or microswitches can trigger the control module to activate the alarm. Without the necessary power getting to and from the sensor, the control module reads it as open, which is interpreted as someone attempting to break in.

This is especially common when the wires are under a lot of stress and movement, such as door hinges. However, it can happen in other parts of the car, and the cause may be corrosion, physical impact, or rodents chewing on the wires.

If this is the culprit, the problematic wiring must be repaired and isolated properly for the alarm to work. Besides the wires, also check and clean the connectors on different alarm components.

7. A Faulty Alarm Control Module

TWB #71 | Honda Car Alarm Module Teardown

An alarm control module with an internal fault can accidentally misread the data it gathers from all the sensors and microswitches and trigger the alarm seemingly at random. The module is essentially the brain of the car. If it’s accidentally bumped or suffers an internal short in the circuiting, it can trigger the loudspeaker and notify the nearby people.

Unfortunately, you’ll likely have to replace the entire unit in this situation. However, this might be as simple as purchasing the same aftermarket car alarm or simply replacing it with a new module for the same make and model of car.

8. Improper Mounting

Improper Mounting

An installation mistake or loose mounting hardware that jostles the control module can cause the car alarm to go off, seemingly for no obvious reason. Sometimes, the module wasn’t wired correctly, the sensors weren’t installed properly, or some components were damaged while being mounted.

This is especially likely if you installed an aftermarket car alarm. However, a new car can have a manufacturer’s fault in the installation. Therefore, only mounting your alarm if you know what you’re doing is recommended. In case you’re unsure, it’s best to get a professional.

How Do I Turn My Car Alarm Off?

If your car alarm keeps going off at random, there are several things you might try to get it to stop. However, this is only temporary until you find and fix the issue.

Removing the power from the car won’t always get the alarm to stop, as some alarms have a battery, and they’ll continue to have power even if the battery is removed.

1. Turning the Alarm Off with Your Key Fob

Turning the Alarm Off with Your Key Fob

This is the first thing you’ll want to try, and most times, it will get the alarm ringing to stop before it’s activated again. Since the alarm is activated when you’re away from the car, telling the car you’re ‘here’ will stop the alarm.

Press the unlock button on the car’s key near your vehicle. You have to make sure your key is functioning and has a battery. If your battery needs to change, check your owner’s manual for instructions on replacing the key fob batteries.

2. Pull the Car Alarm Fuse Out

Disable or quick fix Car alarm going off

If the alarm keeps going off randomly every time you reactivate it, then pulling the car alarm fuse will give you peace of mind while you figure out what’s wrong.

Your car’s fuses can be found in locations such as behind the glove box, below the dashboard, under the hood, or in the cargo. Fuse boxes usually have a diagram indicating what each fuse protects. If not, you can find this info online for your car model and year.

Find the appropriate fuse and pull it out with pliers. If the alarm goes silent, you’ve pulled out the correct fuse. Make sure to return the fuse after fixing the issue; otherwise, the alarm won’t work when it should.

3. Turn the Alarm Off by Removing the Control Module

Tips for removing a car alarm system

Removing the car alarm control module is the method of last resort and takes the most effort. However, this also means you’re permanently disabling the alarm.

Locate the alarm control module. Schematics and diagrams for your model can help if your alarm is OEM. The alarm may be hard to reach and remove. This is by design to make it hard for a would-be car thief to mess with it.

What To Do If Your Car Alarm Keeps Going Off

What To Do If Your Car Alarm Keeps Going Off

First, you’ll want to ensure the alarm is properly turned off so the noise isn’t distracting you from diagnosing the problem. So, it’s best to turn off the alarm temporarily. Ensure all the components are there; otherwise, the diagnostic tool won’t relay the correct info.

If you’re unsure what to do, having a professional check your car is best.

If you’re fixing yourself, you should plug in an appropriate diagnostic tool and check for errors. The alarm will sometimes be an independent unit within the diagnostic. These errors will point you in the right direction toward any of the flaws described above.

The errors will not always tell exactly what’s wrong, and you’ll have to put some thought and experimentation into diagnosing the issue. For example, if there is an open circuit error for one of the sensors, it can be a broken wire, but it can also be a missing sensor or a faulty connector.

Based on the error it gives you, you can try some of the following:

  • Measuring the battery voltage and charging or replacing the battery if it’s insufficient.
  • Checking and repairing the wiring connected to the sensors
  • Cleaning the connectors
  • Cleaning, repairing, or replacing the latches is reported as open in a diagnostic tool or dashboard.
  • Changing the sensitivity of the sensors
  • Repairing or replacing the control unit

FAQs

Will the car alarm eventually stop?

Technically, yes, but you can’t just rely on this, hoping it will stop anytime soon. The alarm can stop after a certain period of time, which could be from 20 seconds to as much as 5 minutes. However, the issue will turn on again if it is still present.

Since some alarms have their own battery, they won’t stop ringing until this battery drains out, and that can take a very long time since not much power is required for the speaker.

Do car alarms deter thieves?

Yes! Even if they aren’t foolproof, car alarms can and will deter many thieves and perhaps even save your car. If thieves see a car with an alarm, they’ll be less likely to steal it. And even if they decide to try, they can activate the alarm, which, in many cases, will make the thief run so they won’t be discovered and notify any people nearby.

It’s better to have an alarm than not to have it; it won’t guarantee that your car doesn’t get stolen, but it will significantly reduce the chances.

Conclusion

Having your car alarm constantly going off randomly can be very frustrating. Figuring out what’s causing it starts with deactivating the alarm to get some peace of mind and keep the neighbors from getting upset.

Then, take a moment to ensure all the doors are fully closed and the hood latch or other panels are properly latched. Take a moment to check the battery’s charge and look for any corrosion on the terminals.

If the car alarm still goes off after rearming it, then you know it’s a more severe electronic issue. Hooking up a code reader or similar diagnostic tool might give you insights on whether it’s the wiring leading to a sensor, a sensor set too lightly, or a faulty control module that needs replacing.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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