(Updated on June 17, 2021)
You may have arrived at this page listening to your own car’s piercing siren blaring in the background after frantically Googling for things like, “Can a low battery cause a car alarm to go off?”
Even though you’ve checked and found no hoodlums around your car, the alarm persists. If you can’t turn it off with a key fob, read the section about how to turn the alarm off first.
If you keep finding yourself in this predicament (often in the middle of the night, when people are sleeping), it’s helpful to understand how a vehicle security system works, some possible causes of an alarm that won’t stop going off, and how to fix the problem to avoid these rude interruptions.
How Does a Car Alarm Work?
Although there are many possible vehicle security systems, the basic design is the same.
A computer (sometimes the car’s ECU, sometimes a separate module for the alarm system) receives input from sensors throughout the car and from the receiver. The receiver “hears” commands from a remote key fob via radio signals and responds by directing the vehicle’s battery-powered loudspeaker to emit the alarm sound when needed.
The security system sometimes has a backup power source in case a potential thief cuts the battery cables to try to make a quiet getaway.
Basic alarms sense a big drop in voltage, like if a door is opened, an attached trailer with an electronic connection to the car is detached, etc. The computer can then react.
What Can Trigger a Car Alarm?
Door/Trunk/Hood Latch Sensors
Newer cars have more sensors than older cars, though most alarm systems have at least door, trunk, and hood latch sensors to tell when these are opened.
These sensors use a spring-activated button or lever to close the circuit (which lets the power flow) when the door opens. This power illuminates the dome light and sets off the alarm, if the computer deems this necessary.
Shock sensors detect movement of the car by using metal contacts that touch one another when the car is sufficiently bumped to complete the circuit.
More advanced systems can determine the severity of the bump as well, leading the computer to respond appropriately. Sometimes this means that if a passerby bumps into the vehicle they’re given a little honk of the horn and loud wailing if they hit the car with a crowbar.
It’s very helpful to have a system that can tell the difference between big and little bumps. Sometimes a sensitive or basic alarm is set off by a cat jumping up onto the hood, which will probably wake up the entire neighborhood.
Sometimes very loud noises can set off the shock sensors, because of the sound waves pummeling the vehicle.
Some thieves are more clever and will simply tow the vehicle away to avoid triggering the alarm system. More advanced alarms, however, have found a way to deal with this.
Tilt sensors detect when the car is tipped by using mercury as the conductive metal liquid to close the circuit.
Even if the ambient air pressure inside and outside the vehicle are the same, breaking a window briefly increases the pressure inside the cabin. Pressure sensors detect this small change in pressure and signal the computer.
Sometimes the sensors are a separate device, but in some cars the built-in speakers can actually be used to “hear” the pressure change.
Speakers make sound by moving back and forth to vibrate the air, creating sound waves. Therefore, in reverse the sound waves move the speaker and the wires send a signal.
These listen to sounds and have been programmed to only notice and signal for breaking glass.
These are really only used in very advanced systems, like in expensive vehicles. They can detect when someone is too close to the car and give a pre-recorded verbal warning.
Reasons Your Alarm Is Going Off
A car alarm that goes off randomly with no rapscallions around can have another underlying issue.
1) Malfunctioning Key Fob
Since the key fob sends commands to the car’s computer relating to the alarm system, a fob with issues can send false alarm signals.
First, try replacing the battery in the key fob if you haven’t done so recently. It’s a good idea to change the batteries once every year or two just in case. The owner’s manual for your vehicle should include a section on the exact battery replacement interval you should expect.
If the problem persists and you suspect it’s the key fob, try resetting the fob. You may need help from the manufacturer to do this or the procedure instructions might be in the manual.
The car can also be hooked up to an OBDII reader to scan for codes. This can be done at most auto parts stores for free. Not all key fob issues will throw a code, but it’s worth trying.
2) Low or Dead Battery
The car alarm is not only used to deter thieves or find your car in a gigantic parking lot! It can also be the alert from your car to you to tell you when your battery is low. If the alarm goes off while you’re starting the car, it may be trying to tell you something.
Check the voltage on your battery with a voltmeter. If the charge is at least 12.6 volts, the battery isn’t the problem.
3) Corroded, Rusty, or Dirty Battery Terminals
If the charge can’t flow properly from the battery to the cables, the computer might interpret that as “low battery” and alert you. Of course, the terminals should be clean for optimal function and lifespan of your battery. If there is any debris on the terminals, disconnect the battery and clean them.
4) Debris on Hood Latch Sensor
While all sensors can get dirt and gunk on them, it’s most likely to happen on the hood latch sensor because of its location at the front of the car. This can cause a false alarm because the computer may interpret debris on the sensor as an open hood.
If there is oil, grime, or dirt on the sensor, try cleaning it by gently scrubbing it with brake fluid and drying with a microfiber cloth. If the problem persists, the sensor may need to be replaced.
5) Dirty or Malfunctioning Door Lock or Trunk Sensor
These may be trickier to access as they are inside the door or trunk, but you may be able to check the voltage with a multimeter to see if any of them are causing false alarms.
6) Wiring Problems
If one of the sensors is not properly connected to the computer, signals can get weird. First try disconnecting and reconnecting the hood latch sensor as that is often the culprit. If the other sensors are accessible you can do the same for those.
7) Incorrectly Installed Alarm System or Faulty Alarm Module
The alarm module is the security system-specific computer. Some drivers prefer to install an aftermarket alarm system, but either those or factory systems can be installed incorrectly.
8) Shock Sensors Are Too Sensitive
If the sensors are programmed to care too much about smaller bumps, you may be having more false alarms. Sometimes it’s possible to reconfigure or reprogram the alarm system yourself to decrease the sensitivity – check the manual.
You may be able to reprogram it by plugging a diagnostic tool into the OBDII port or there may even be a dial underneath the dashboard somewhere.
How To Turn the Alarm Off
Assuming you’ve tried the key fob alarm button and it didn’t work, there are a few strategies to try to stop that terrible shrieking.
1) Turn On the Car
Sometimes simply getting into the car and turning it on with your normal key or fob is enough to stop the alarm.
Try using the lock and/or unlock buttons on the key fob to remotely stop the alarm. If this doesn’t work, use the physical key (every key fob should have one tucked inside) to lock and unlock the car door.
3) Shut-Off Switch
This is probably only an option in some aftermarket alarm systems, but there may be a switch hidden near the driver’s feet to shut off the alarm.
4) Disconnect the Fuse
If none of the other things worked you may just need to disconnect the fuse to cut the power to the alarm. Check the manual for fuse location in a factory system, but in most aftermarket systems the fuses are too hard to access.
5) Reset the Alarm System
It is sometimes possible to reset the alarm system if it seems haunted by electrical ghouls. Check the manual for this.
6) Disconnect the Battery
When all else fails, disconnect the battery (always remove the ground terminal first; usually it’s negative). If there is a backup power supply the alarm may continue to wail until that is drained.