Subaru Legacy Alarm Going Off? (8 Common Causes)

The Subaru Legacy is a popular midsize sedan.

Similar to other vehicles, its alarm may be triggered randomly and for unknown reasons.

In the event that your Legacy’s alarm has been going off, this article will help you to figure out the cause and prevent it from happening in the future.

Why Does My Subaru Legacy Alarm Keep Going Off?

The most common reasons for a Subaru Legacy alarm going off include a dying 12V battery, problems with the hood latch sensor, overly sensitive sensors, key fob problems, a dying or dead internal battery of the alarm, a faulty aftermarket installation or accidentally pressing the panic alarm.

1. Dying 12V Battery

A 12V battery that is weak, dead, or dying will cause the Subaru Legacy’s alarm to go off randomly.

If you suspect that your battery may be causing the problem, it is best to have it tested at a local parts store.

They will be able to replace the battery if need be.

A 12V car battery has a lifespan of approximately 3 to 5 years, so it might be time for a new one.

Before you run to replace the battery, it is a good idea to check the battery’s connections yourself.

Ensure that your connection points are clean and all connections are tight.

To clean the terminals, a proven trick is to mix a solution of baking soda and water.

Use a teaspoon with a small amount of this solution and an old toothbrush dipped in this mixture to scrub the terminal points clean of dirt, grime, or build-up.

2. Broken, Damaged, or Dirty Hood Latch Sensor

The hood latch sensor is situated under the hood near the grille and is designed to detect whether the hood is open or closed. If the sensor is broken, damaged, or dirty, it can trigger a false alarm.

The hood latch sensor forms an integral part of the alarm system of your Subaru Legacy.

If the hood latch sensor is malfunctioning, this will cause the alarm to go off at random because it is most likely feeding incorrect information back to the car’s computer and alarm system.

Essentially, it thinks someone is

However, you may just need to give the hood latch a good clean and some lubrication using WD-40.

3. Oversensitive Alarm Shock Sensors

Some Legacy models are fitted with shock sensors, which can detect hits and impacts around the car, such as the shock of a window being smashed.

Oversensitive shock sensors can set the alarm system off at random times.

They can be triggered by cats, strong gusts of wind or someone brushing past your car – depending on the sensitivity settings.

A user on the Legacy GT forum had this to say:

“My alarm goes off when my neighbor in his Volkswagon without an exhaust drives by my parked car an hour before I have to get up for work… It has a stock vibration sensor, which will go off from loud/heavy cars and trucks.”

The only way to fix that is for the dealer to adjust the sensitivity levels of the sensors.

4. Key Fob Problems

You may overlook the key fob when trying to diagnose the cause of your Subaru Legacy’s alarm going off, although key fob issues are a very common cause.

For the sake of a few dollars its worth changing the key fob battery, make sure you use a good-quality battery and that it is inserted the correct way up.

It is also important to give the key fob a good clean as dirt and dust can compromise the electrical contacts.

You can use rubbing alcohol and cotton balls to gently lift any dirt.

Once you have put your key fob back together, test it by locking and unlocking the car.

If the alarm still goes off, you may need to head to a dealer or specialist to inspect it further.

One user on shared the following on the forum: “I changed the battery on my key fob and it solved the problem. The alarm has stopped blaring at random.”

That can be the solution to your Legacy’s alarm too.

5. Alarm’s Internal Battery

Older generation Subaru Legacy models were equipped with Sigma alarms with internal backup batteries, which could cause problems with the alarm system.

Much like the Legacy’s 12V battery, the alarm’s internal 9V backup battery could also have dirty terminals, loose connections, or be dying.

Moisture can also cause problems with this small battery.

To locate the battery and the siren, lift the hood and look under the scuttle.

An owner on shared the following info on that forum:

“Also on the sigma when the inbuilt rechargeable pp3 battery in the siren goes bad it can cause constant beeps (can be replaced).”

6. Faulty Aftermarket Alarm Installation

If your Legacy has an aftermarket alarm system installed (meaning one that wasn’t factory-fitted), there’s a possibility that it might have been improperly installed or has faulty wiring.

To avoid such complications, it’s advisable to have a professional handle the installation of an aftermarket alarm rather than attempting to do it yourself.

Aftermarket alarm systems offer more security features (and they are more sophisticated than the standard factory-fitted units) but they can be problematic.

If you have an aftermarket alarm system that’s giving you headaches, it’s best to have an auto electrician take a look.

7. Faulty Door Lock Sensors

A faulty door latch sensor is a common reason why a Subaru Legacy alarm may go off at random.

Similar to the hood latch sensor, your Legacy’s alarm system monitors the doors via sensors to make sure no one is forcing them open.

Give all door latches a good clean and some lubrication with WD-40.

Door latch sensors and door latches will incur a lot of wear and tear due to the door constantly being opened and closed.

Fortunately, these are fairly easy and cheap to replace.

8. Accidentally Pressing the Panic Alarm

Another common reason why some Subaru Legacy owners have experienced their alarm go off at random, is due to accidentally pressing the panic alarm button.

This can be easily done if you have your key fob in your pocket.

On newer Legacy models, the panic alarm is the fourth and final button on the key fob.

Here’s what the manual says:

  • Press the panic alarm button to sound the horn and flash the hazard warning lights.
  • Press any button on the access key fob or transmitter to deactivate the panic alarm.

Alternative Suggestions

Disconnect the Battery

Disconnect the 12V battery connections for 20 seconds as this resets many of the electronics in the vehicle.

It can sometimes get rid of random electrical glitches.

Check For Warning Messages

When the alarm occurs can you see any lights or warning messages on the instrument panel?

This can give a clue as to what’s causing the alarm e.g. ‘Hood Ajar’.

Take it to a Subaru Dealership

If needed, take your Subaru to the dealership.

Tell them you are NOT paying for a check on what the problem might be.

Ask them if they will check it for free.

Most dealerships and other places do quick/initial diagnosis for no money as they plan to make money for the repair of your vehicle.

If you’re car is still under warranty then they should fix and resolve the issue for free.

Check for Recalls or TSBs:

By entering your car’s VIN number on Chevrolet’s recall page or the NTHSA’s Safety Issues & Recalls page you can determine whether or not there is a TSB or recall for your vehicle and if there is you’ll want to get it addressed.

A recall is issued by a vehicle manufacturer for issues that are safety-related, while a TSB covers components that may be malfunctioning but don’t compromise the safety of the vehicle.

Test the 12V Battery

How to Test the Battery

  • Before testing, remove the surface charge from the battery, this allows for an accurate reading.
  • Simply turn on the headlights for 2 minutes then turn off.
  • Set the multimeter dial to the ’20 Volts’ setting.
  • Make sure the car is turned OFF

The multimeter will have a red probe and a black probe:

  • The red probe is for making contact with the positive terminal
  • The black probe is for making contact with the negative terminal.

Measure across the battery terminals.

  • The meter should display a reading, if the battery is fully charged the voltage should be between 12.2 and 12.6 volts.
  • Anything under 12V and the battery should be charged or replaced.



  • Ian Sawyer

    Growing up with a father who was a mechanic I had an appreciation for cars and motorcycles from an early age. I shared my first bike with my brother that had little more than a 40cc engine but it opened up a world of excitement for me, I was hooked. As I grew older I progressed onto bigger bikes and...