The Subaru Impreza is a popular choice thanks to its affordable price tag.
However, just as with any other car, its alarm may go off randomly and for unknown reasons.
If your Impreza alarm keeps going off, this article is here to help.
Why Does My Subaru Impreza Alarm Keep Going Off?
The Subaru Impreza’s alarm can be triggered due to a dying 12V battery, a broken hood switch, problems with the key fob, the alarm’s internal backup battery, electrical interference, faulty door latch sensors and faulty aftermarket alarm installation.
1. Dying 12V Battery
A dying 12V battery is the most common cause of an Impreza alarm going off. A battery that is dying or has insufficient voltage can trigger the alarm, as well as cause several other problems in your car.
Most 12V car batteries last 3 to 5 years, so depending on when you purchased the vehicle or swapped the battery, it might be time for a replacement.
Before swapping your car’s 12V battery, it is worth taking the time to disconnect the battery, clean the battery’s terminals, and then reconnect the battery – making sure that the connections are tight and there are no obstructions or dirt and debris on the connecting points.
You can clean the terminals using a toothbrush dipped in baking soda and water mixture.
If you have a multimeter in your garage, you can do a battery check yourself. Other than that, take your car to your nearest AutoZone – most offer free battery health checks.
One owner on the scooby.net forum shared their experience:
“Car battery for me as well. I hadn’t used the car for a while, then woke up at half 2 in the morning with the alarm going off. Had to go into my garage, get a spanner and disconnect the battery. Charged it the next day, and all has been fine since! (Car back on the road)”
2. Broken, Damaged, or Dirty Hood Switch
A broken, damaged, or dirty hood switch is the second most common cause of a Subaru Impreza alarm going off. If a hood switch is faulty or dirty, it can trigger the alarm as a hood that moves can simulate a break-in and thus set the Impreza’s alarm off.
The hood switch is a pivotal part of the Impreza’s alarm system.
Once the car alarm has been armed and the hood is shut, the hood switch relays to the car’s computer that the car is safe and secured.
In instances when the hood switch is broken, damaged, or dirty, this can interfere with and interrupt this electrical signal and can cause the alarm to go off at random times – thinking that the hood is being opened in the event of a break-in.
Dirt and debris can clog up the hood switch, so thorough cleaning and lubrication of the latch can prevent and stop the alarm from going off at random times.
Another poster on the Scooby.net forum shared this:
“I had the same issue on my hawkeye recently when the weather got warmer. Had a good look round it and noticed hood switch mount had been slightly bent backwards. Bent it straight and been perfect since (touch wood). My only guess is a careless mechanic must have knocked it and sun hitting the car made the hood expand? Dunno. either way seems to have cured it.”
3. Key Fob Problems
Another common cause of the alarm going off in your Impreza can be a faulty key fob. A faulty, dirty, or damaged key fob can cause your Impreza alarm to go off at random times too.
Open your key fob to clean its internals and rid it of any grit and grime that may have built up inside. You can use some rubbing alcohol and a soft cloth to clean the contacts.
If the battery is acting up, now is also an excellent opportunity to replace that. They’re fairly cheap and readily available – we recommend getting a good-quality battery.
Once you’ve reassembled your key fob, and tried using it to lock and unlock your car, but the alarm still goes off after using the key fob, you may need to take it to a dealer or a specialist.
Does your spare key fob work? If that is good, then you should consider having your original key fob reprogrammed or inspected by a dealer or vehicle key specialist. They can reprogram your key fob with the code for your alarm system, thereby resetting your key fob.
Here’s someone’s solution from the Scooby.net forum:
“Mine did this the other day, I changed the battery in the keyfob and it’s not done it since.”
4. Alarm’s Internal Battery
On some older Subaru Impreza models, the alarm has a battery backup – if yours does, then you might need to consider inspecting the alarm’s internal battery.
Just like the 12V car battery, the alarm’s internal backup battery may have dirty terminals, loose connections, or be dying. Moisture or water ingress may even be the culprit.
With the hood up, locate the alarm and the siren; these are under the windscreen scuttle panel.
You’ll need to remove the siren and check for water, dirt, or grime buildup.
The backup battery for the siren may need replacement.
One way to check is with a voltmeter or a multimeter.
These are usually 9V batteries, and you can purchase them from AutoZones or other car parts suppliers.
One poster on the Scooby.net forum explained the process:
“It will be the internal battery in the siren under the scuttle. There could possibly be water ingress too so the circuit may be fried. Wipers off, scuttle off, IIRC it’s a 10mm bolt to remove the siren, split the unit check for water. As a temporary measure, you can just disconnect the plug to the siren, the alarm will still function just not sound. I had the same issue, sourced a siren from a breakers for £15 [$11]. The replacement siren had sealant inside the unit where it enters the siren casing and the mounting bracket is slightly different. Wonder if they have been manufacturer-modded due to water ingress issues?”
Another shared more insight into this common problem:
“AFAIK, the alarm backup battery is in the siren module located under the bonnet rather than in the actual alarm ECU unit.”
5. Faulty Aftermarket Alarm Installation
If your Impreza has been fitted with an aftermarket alarm system (that is, one which did not come as factory-fitted with your Impreza), then it may be incorrectly installed or have bad wiring.
For this reason, it is always good to have an aftermarket alarm installed by a professional.
Some aftermarket alarm systems offer more security features and are more sophisticated than the standard factory-fitted units.
If this is the case, have it inspected by an auto electrician who specialized in alarm systems.
6. External Electrical Interference
There are several reports on forums of owners complaining that their Imprezas’ alarms are triggered by electrical interference.
This can include parking near HAM radios or underneath high-voltage power lines.
Even nearby microwaves can play havoc with your Impreza’s alarm.
This is how one owner on Scooby.net found out how his alarm went off:
“I realized it was only ever at home this was happening then it was only if it was the first car in the drive nearest the house/kitchen. Then I was making something in the microwave one day and as I hit start the car alarm went off!!! Tested this a few times and hey presto that was the problem the microwave was interfering with the alarm signal!!! Phoned the supplier who were over the moon we had got to the bottom of it. Never parked next to the kitchen door again problem solved!”
Once you’ve deduced that this is the trigger for Impreza’s alarm going off randomly, park it elsewhere to avoid the frustrating and annoying problem.
7. Faulty Door Lock Sensors & Sticking Latches
A faulty door latch sensor is a common cause of an Impreza alarm going off, in which case the faulty sensor will need to be replaced.
Similar to the hood latch sensor, your Impreza alarm monitors the doors to make sure no one is opening them.
Give all door latches a clean and some lubrication with WD-40 first to see if that solves the problem.
A latch that is sticking can also trigger the alarm.
Disconnect the Battery
Sometimes mysterious alarm problems can disappear with a simple reboot, there are no guarantees here but it’s worth a try.
Disconnect the battery for 20 seconds and this resets many of the electronics in the vehicle.
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 Impreza 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 Subaru 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 Subarus 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.