8 Reasons Your Car Unlocks Itself Overnight!

(Last Updated On: February 1, 2024)

Have you ever woken up in the morning to find your car unlocked despite being sure you locked it the night before? I bet you let something valuable in there too! 

The most common reason a car unlocks itself overnight is the key fob malfunctioning; the internal circuit board fails, and it sends signals to the vehicle when it is close enough to trigger it to open. Remember, most of the time, your car is well within the proximity of the key when parked on your drive overnight. 

In this article, I’ve explained the eight reasons I’ve come across when checking why a car may be unlocking itself overnight and provided some tips on diagnosing what’s causing this issue. 

8 Reasons Why You Car Unlocks Itself Overnight

When identifying which of the eight reasons below is causing your car’s issue, pay attention to the other symptoms because that will highlight exactly what’s causing the problem. 

Key Fob Malfunction

If your car unlocks itself overnight, the key fob failing is the most common reason. This can happen when the buttons on the key fob get stuck, the battery is low, the internal electronics have failed, or the sensors fail to detect the presence of the fob. When this happens, the fob will send erratic signals to the car’s computer system that trigger the automatic unlocking mechanism. Equally, the car may also lock itself randomly.

The key fob is generally close enough to the car to unlock if you press the button when it’s in your house, and the vehicle is parked on the drive or in the garage. One quick way to prove it’s the key fob is to move it far enough away that the key fob will no longer unlock the car and leave it overnight. If it doesn’t unlock like it has been, you have found the problem. 

Wiring Issue 

A wiring issue can sometimes cause the lock actuator to open the door or signal the control unit to unlock when it shouldn’t. This can be due to a short circuit or a damaged wiring harness, causing the car’s computer system to unlock the vehicle on its own.

Sometimes, if there is a problem in the car’s electrical system, the vehicle will reopen right after you lock it. You might not notice it immediately, but see it’s still open the following day. Additionally, if you have an electrical wiring issue, you might experience your car randomly locking and unlocking while driving.

Automatic Unlocking Feature

Most cars today have an automatic unlocking feature activated when the key fob is within a certain distance of the vehicle. However, this feature can sometimes malfunction, causing the car to unlock itself even when the key fob is not nearby. This can happen due to a malfunctioning sensor that detects the key fob’s proximity to the car.

Simply trying the door handle after closing it will confirm this. However, this won’t work on a proximity key; it will automatically reopen if it is nearby. If you have a proximity key, you’ll need to move it out of its ‘proximity,’ usually 30 yards or so, and then try the door. 

Remote Signal Interference

If interference in the radio signal between your key fob and your car, it can cause your vehicle to unlock itself. This can be due to various factors, such as a nearby radio tower, another electronic device, or even bad weather. Additionally, metal objects or walls can sometimes block the key fob’s signal, stopping it from locking in the first place. 

You can easily confirm signal interference by parking the car somewhere else overnight and seeing if it unlocks itself. Specific car models experience more frequent remote signal interference, which car thieves exploit to unlock vehicles without the key fob.

Faulty Actuator

The actuator is the mechanism for moving a rod or cable, which locks or unlocks the door when activated. However, when the actuator malfunctions, it can cause the doors to lock and unlock randomly. This usually happens due to a short circuit or a damaged wiring harness.

You may notice other issues with a faulty door lock actuator, such as a clicking noise when trying to lock or unlock the door without it actually locking or unlocking.

Failed Door Solenoid 

A solenoid is an electromagnet that operates the locking mechanism in the car’s door. It consists of a coil wire wrapped around a metal core, and when an electrical current passes through the coil, it creates a magnetic field that moves the metal core, activating the door lock mechanism. Over time, the solenoid can wear out. When the solenoid fails, it can cause the doors to lock and unlock randomly, which can lead to the vehicle unlocking overnight. 

In addition to the car unlocking overnight, a failed door solenoid can cause other problems with the car’s locking system. You may notice that

  • the door locks fail to respond when you use the key fob, or the doors lock and unlock very slowly. 
  • Hear unusual noises, such as clicking or grinding, when locking or unlocking the doors.
  • The door may fail to lock or unlock. 

Failed Door Sensor

The door ajar sensor detects whether the door is open or closed. When the sensor fails, it can send the wrong signal to the car’s computer system, causing the doors to unlock. For example, it may be fooled into thinking it is wide open, preventing it from locking when it is shut. The sensor usually fails due to wear and tear but can also fail from repeatedly slamming the door. 

In addition to the doors unlocking unexpectedly, a failed sensor can cause a few other symptoms. For example, you may notice that the interior lights remain on even after you’ve closed the door or that the door locks don’t respond as quickly as they should, or the alarm won’t set. In some cases, the sensor may also fail to detect when the door is closed, causing warning lights to remain on the dashboard

User Error

Now, I don’t want anyone to take offense here, but are you pressing the button to lock the doors and not assuming it locks itself? I’ve also seen the spare key fob left inside a vehicle, and it was a proximity key, so the car is unlocked as soon as the door handle is touched. It’s not actually unlocking itself, I know, but it was never unlocked in the first place. 

faulty key fob

How to Diagnose What’s Causing the Car to Unlock Itself?

Diagnosing the cause of your car unlocking itself can be tricky. Using the information above, here are some steps you can take to figure out what’s going on:

  1. Check the key fob – A malfunctioning key fob is one of the most common causes of a car unlocking itself overnight. Check the fob’s battery and ensure the buttons aren’t partially pressed.
  2. Check the electrical system to see if all other electrical systems in the car are working. Is the door ajar warning light on when the doors close? Are there clicking noises when trying to lock the door, but the door won’t lock? 
  3. Check the automatic unlocking feature – Some cars have an automatic unlocking feature that conditions such as being within so many feet of the cause it to unlock. Check the handbook to see if this can be disabled to see if it solves the issue. 
  4. Check for remote signal interference – A nearby radio signal may interfere with your key fob’s signal, causing your car to unlock itself. Try parking your car in a different location or shielding your key fob by wrapping it in aluminum foil overnight.
  5. Check for user error – It’s always possible that you forgot to lock your car in the first place. Make sure you’re locking your car properly before assuming there’s an issue.

Final Thoughts

As you can see, many potential causes of a car unlocking itself overnight exist. You should be able to diagnose and fix the issue by following the steps above. If you’re still having problems, it’s best to take your car to a professional mechanic to have it looked at. Don’t leave your car unlocked and vulnerable to theft.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.