The Prius is one of the most popular hybrid cars ever made.
Like any other vehicle, it’s not uncommon for the alarm to go off at random.
If you’re Toyota Prius alarm keeps going off this article is here to help.
Why Does My Toyota Prius Alarm Keep Going Off?
A Toyota Prius alarm can be set off due to a faulty key fob. Other common causes include activation of the interior motion sensors, a dying 12V battery, a faulty hood switch, faulty door sensors, a low key fob battery, wiring problems or a faulty body control module.
If your Prius alarm is going off when it shouldn’t then by process of elimination, it shouldn’t be too hard to narrow down the root cause.
1. Faulty Key Fob
A faulty key fob can cause problems with the alarm system. Try using a replacement key fob to see if the alarm problems stop.
Here’s what owners had to say on the forum PriusOline.com:
“I have finally fixed the problem after putting up with it for 8 months. I tried everything only to find out it was the key fob!! Replaced it with backup fob and haven’t happened since. Seriously the alarm went off at least once a day alost 24 hours to the minute. Some days more.”
“I went to Toyota Service, I did not tell them about the alarm problem, I just asked them to get a new FOB which was very expensive by the way. Since then, the alarm has not gone off again for the last 2 months. Curious thing is that they gave me the old FOB working as well. So, I would recommend telling the service to repair your FOB, it might be a lot cheaper than buying a new one.”
2. Interior Motion Sensors
A common cause of the Prius alarm going off is the triggering of the interior motion sensors. If your model Prius has a “Theft Sensor” button inside the vehicle this button will turn the interior motion sensors on and off.
On the second-gen Prius there is a button below the steering wheel to turn off the interior alarm sensors.
The idea behind these is that it lets you keep your sunroof or window open – but the alarm will go off if someone reaches into your car to steal something like your phone or iPad.
Sometimes insects and bugs might get trapped in the car or even leaves being blown in, which can be picked up by these sensors thereby triggering the alarm.
- You can try disabling the interior sensors for a week and see if this is what is causing the alarm to go off.
- You can also try fumigating the car with bug spray to kill any insects, bugs or moths which might be triggering the alarm.
The intrusion sensor (interior motion detector) is on unless you deliberately turn it off by pressing the button before leaving the car and locking it.
It doesn’t stay off next time you lock the car unless you set it off again.
If you turn it off, you can change your mind by pressing the button again (so don’t press the button twice just to make sure).
3. Low Key Fob Battery
If your Prius has a low key fob battery this can trigger the alarm to go off at random.
Try using your spare coded key – if the problem goes away then you know you’ll need to replace the battery in your primary key fob.
It’s advised that you don’t carry big metal objects, electronics or a second coded key on the same keyring as your primary key fob as this can lead to problems starting your Prius.
It might also be worth giving the inside of your key fob a clean as these can get filled with dirt which could be causing the alarm issue.
4. Dying 12V Battery
A dying 12V battery or a battery with insufficient voltage can cause a wide range of problems, including triggering the alarm.
Most 12V car batteries last about 3-4 years so it might be time for a replacement.
It’s worth cleaning the terminals first though and making sure the connections are tight and free from dirt and debris.
Clean the terminals using a toothbrush dipped in baking soda and water mixture.
The next step is to check the battery, you can do this with a multimeter or take your car to any AutoZone who often offers free battery health checks.
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.
5. Faulty Hood Latch Sensor
A common cause of the alarm going off on the Toyota Prius is due to a faulty hood latch sensor otherwise known as a hood switch.
Due to their location, hood switches often get dirty and clogged up so it’s worth giving it a clean first.
The hood sensor is a simple electrical switch, that monitors whether the hood is open or closed (it’s a small rubber stump that’s black).
You may even notice a hood ajar message pop up when you’re driving.
If the sensor completely fails you may see the hood ajar message constantly on.
If you suspect the hood switch is faulty and your car is still under warranty we recommend taking it to the dealer and having them replace it for free.
If you’re mechanically inclined you can simply replace the hood switch yourself, these can be picked up for $40 – $80.
6. Corroded or Rusty Battery Terminals
If your Prius has rusted battery terminals it will be unable to deliver the correct electrical power to various parts of the car.
The alarm system will often interpret this as a low-battery scenario and trigger the alarm.
Rusting can be caused by moisture and road salt exposure but also by improper charging.
- When a battery is undercharged it is common to see corrosion form on the negative terminal.
- Similarly, an overcharged battery will see corrosion form on the positive terminal.
Corroded battery terminals and posts can be cleaned by applying baking soda and scrubbing with a wet toothbrush.
If the battery terminals are severely rusted you will need to replace the battery.
7. High Voltage Power Lines
Parking underneath overhead power lines can trigger the alarm on your Toyota Prius.
This phenomenon is caused by electromagnetic radiation (EMR) which can interfere with the electronics on your alarm system.
If you suspect this is the cause, simply park somewhere else out the way.
Depending on where you live this may not be relevant as power lines can often be located underground.
8. Faulty Interior Motion Sensor
Sometimes faults develop with the interior motion sensors, this can cause the alarm to be triggered at random.
Although this is less of a common occurrence it can and does happen.
9. Faulty Door Lock Sensors
A faulty door latch sensor is a common cause of Prius alarms going off.
Similar to the hood latch sensor, your Prius alarm monitors the doors to make sure no one is opening them.
It’s a good idea to give all door latches a clean and some lubrication with WD-40.
If you suspect the door lock sensor is faulty and your car is still under warranty we recommend taking it to the dealer and having them replace it for free.
10. Faulty Body Control Module
A common reason why a Prius alarm keeps going off is due to a faulty body control module.
The body control module or ‘body computer’ is the electronic control unit responsible for monitoring and controlling various systems associated with the vehicle’s body such as the alarm, immobilizers, power windows etc.
The body control module can develop corrosion on the pins or connections can become loose.
You can typically pick one up for around $650 and if you’re not mechanically inclined it’s probably best to have someone at Toyota fit it for you.
Other common symptoms of a bad BCM include:
- Repeated battery drain
- Starting problems
- Erratic electrical functions e.g. horn, wipers, lights, lights on the dash
- Security and alarm system problems
11. Damaged Wiring
Damaged electrical wiring can cause a wide range of problems.
Wiring damage can occur from general wear and tear or even from rodents chewing on the wiring.
Broken wires aren’t easy to find and you’ll need to have an auto electrician carry out some basic tests on your vehicle.
A common point of failure for electrical wiring is in and around the doors.
12. Incorrect Installation of a New Alarm
If you have recently had a new alarm fitted and it’s going off at random, then there’s a good chance it was installed incorrectly.
Your best option is to go back to the mechanic who installed it and explain your problem.
13. Aftermarket Alarms
If the car has an aftermarket alarm fitted (one that did not come as standard with the vehicle) then this may have been incorrectly installed.
It may also have overly sensitive sensors which can be triggered by strong wind or even a cat or dog.
These alarm systems are often more sophisticated than a basic factory-installed car alarm but are often installed by people who aren’t mechanics.
If you have an aftermarket alarm that’s causing you problems it’s best to have it examined by a trained auto electrician.
Disconnect the Battery
Sometimes mysterious alarm problems can disappear with a simple reboot, there are no guarantees here but it’s worth a shot.
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 gauge cluster?
This can give a clue as to what’s causing the alarm e.g. ‘Hood Ajar’.
Take it to a Toyota Dealership
If needed, take your Toyota 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 Toyota 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 Toyota’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.