7 Causes Your Anti-Theft System is Keeping Your Car from Starting

In the 1990s and early 2000s, aftermarket car alarms caused a lot of fuss and bother. Eventually, automakers caught onto the demand for anti-theft systems and started including them in standard models. Some are simple alarms that go off if the car is struck shaken, or a door is opened while the system is on.

Some of the best anti-theft systems will lock out the engine or arrest the electrical portion of the ignition process anytime they detect a problem. That’s all fine and good when a criminal tries to mess with your car, but what happens if something goes wrong and the anti-theft system won’t let you start your own car?

Most of the time, you can deactivate the alarm with the key fob controls. Though this isn’t always the case, or perhaps you don’t have the fob on you and just have the key. In a time like this, most cars have a way to deactivate the anti-theft system. However, automakers don’t make it easy on purpose.

What Triggers the Anti-Theft System in a Car?

Your car’s anti-theft system is connected to a series of shock sensors and latch sensors. Anytime one of them is triggered, the entire system engages, and usually, the alarm will go off. Even after the alarm stops making noise, the anti-theft system might still prevent the engine from starting.

The shock sensors are designed to activate when the car is jostled or impacted, such as someone trying to break a window. They have different levels of sensitivity. If yours is set a little too sensitive, it’s possible for you to accidentally trip one of the shock sensors by something as simple as banging the car door with a bag of heavy groceries.

Most vehicle anti-theft systems also have latch or lock sensors. There’s usually one on each door, as well as the rear hatch or truck, and another one on the hood latch. These sensors have a minor yet constant electrical draw going through them when the anti-theft system is on. If the door or the hood is opened, it interrupts the circuit and triggers the anti-theft system.

Some super-sophisticated car alarms have motion sensors and audio sensors that pick up the surrounding environment. These have their own set of tolerances that will engage the anti-theft system.

With all these different components and electrical connections, several different things can set off a car alarm or anti-theft system, leaving you wondering what to check.

What Causes Anti-Theft System Malfunctions?

Anti-theft system malfunctions that cause it to come on and stay on can be linked to various internal faults and external conditions. This could be a shock sensor that’s set to sensitive, and an acorn landed on the car, activating it. It could also be something keeping one of the various latch sensors from completing the circuit. Sometimes, it’s just a plain old battery problem.

1. An Overly Sensitive Shock Sensor

An Overly Sensitive Shock Sensor

If just one of the shock sensors on your anti-theft system is set too sensitive, then the slightest thing can set it off. A twig breaking off a tree or the pizza delivery guy brushing up against the car. If you turn the alarm off and tap the car on a door panel with your finger and it goes off, you’ll need to contact the dealership to have them recalibrate the sensor.

2. A Faulty Latch Sensor

A Faulty Latch Sensor

The doors, trunk/hatch, and hood typically have their own type of latch sensor that registers it’s closed by completing the circuit. If something simple like a leaf or a piece of junk mail gets stuck when you close the door, the system will read it and activate the anti-theft system. Rust on the hood latch is also a common fault that can cause this.

3. A Dying Car Battery

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If your car battery is dying or the connections are severely corroded, the anti-theft system self-monitoring will detect the fault and activate the system. This is the anti-theft system’s way of altering you to the problem before the battery completely dies and renders the system inert.

You can test your car battery yourself with a multimeter. Auto parts stores and battery replacement service centers will test your battery for free, but you have to bring the car to them.

It might also be that your alternator is going out, and it can’t charge the battery. So, the wise move is to test both. While you’re at it, make sure your connections are clean and not being hampered by corrosion.

4. The Remote’s Battery Is Dead

The Remote’s Battery Is Dead

If the battery inside your key fob remote is dead, you might press the button to deactivate the anti-theft system, but no signal is actually sent. It remains armed and activates when you open the car door.

5. A Damaged or Engaged Immobilizer Chip

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If the immobilizer chip in the car’s ECU has been damaged or simply worn out, the anti-theft system will prevent you from starting the car. The chip is usually embedded deep in the ECU, and a mechanical fault like water getting on the engine or the engine overheating could damage it.

It’s also possible that someone attempted to break into and start your car, and the anti-theft system stopped them.

In some models, there will be an icon on the dash display that looks like a little blinking key, which tells you that the immobilizer has been activated.

6. Wrong Key

Wrong Key

If you have two cars from the same automaker, like Ford, and you accidentally try to use the wrong key to start your car, it will activate the anti-theft system. The fob itself gives off a faint RF signal that the anti-theft system reads before releasing the immobilizer chip.

In an instance like this, the anti-theft system thinks you are a would-be car thief trying to hot wire the vehicle and lock out the engine to prevent it from starting.

7. A Failed Attempt to Steal the Car

A Failed Attempt to Steal the Car

While it might be distressing to hear, it’s possible that someone attempted to break into your car or tried to steal it, activating the anti-theft system. This usually triggers an alarm, causing them to run away before you even get a chance to see it. Sure, your car is technically safe, but the anti-theft device and the immobilizer chip need to be convinced that you truly are the rightful owner.

How To Shut Off Your Car’s Anti-Theft Device

Shutting off your car’s anti-theft system and getting the engine started might be as easy as pressing the unlock button on the key fob. However, the methods for deactivating an anti-theft system without the fob can vary from one model to the next.

Automakers purposely engineer these systems to be difficult to disengage as a way to deter would-be thieves. Though there are a few simple things you can do, and you might get lucky enough to shut off the anti-theft device without outside help. This starts with inspecting the car to understand what state the anti-theft system is in.

Check & Replace the Key Fob Battery

Check & Replaces the Key Fob Battery

Anti-theft systems are notorious for activating when the key fob battery is dead. It might be that the battery inside the fob is dead, or it’s come loose. If your fob is one that you can open easily, you can replace the battery and try to disarm the anti-theft system again.

Inspect the Door Lock Cylinder

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If a would-be thief tried to jimmy your lock with a screwdriver or a similar tool, it would trigger the car alarm, and they would run off. By the time you get to your car, nothing seems amiss. Yet the anti-theft system remains engaged. Take a moment to see if the lock cylinder is damaged, and check underneath the door handle.

If you find the lock or cylinder is damaged by someone trying to break in, the anti-theft system will remain engaged. In a scenario like this, you can try getting into the car via the passenger side to shut off the anti-theft system. You’ll then have to leave the system disarmed until you can get the damaged lock cylinder repaired. If you activate it again, the anti-theft system will read the affected lock cylinder and immobilize the engine.

Check to Make Sure It’s the Right Key

Check to Make Sure It’s the Right Key

If you accidentally grabbed an identical-looking set of keys for another car and attempted to use it to start your car, it might trip the anti-theft system. This is the car thinking someone is trying to hot wire or jimmy the ignition.

Some cars also have “Valet” keys or secondary keys. They will open doors and such, but they don’t have the same RF transmitting chip. You can identify a valet key, as there will be a V etched somewhere on the key shaft.

Tips for Trying to Start a Car After Someone Else Engaged the Anti-Theft System

Most modern cars with an anti-theft system will lock the engine via the ECU immobilizer chip, which makes it impossible to start the vehicle. There are a few possible ways to disable the system to start your car. This starts with a cursory inspection to see what state the anti-theft system is in.

Check the Anti-Theft Light

Most newer cars have a red or blue dash light that tells you if the anti-theft system is engaged without manipulating the vehicle physically. If the light is on or flashing, it means the anti-theft system is engaged. However, in some cars, the flash only occurs once a minute if the anti-theft is in passive mode. If someone attempted to jimmy the car, and it actively engaged the anti-theft system, it might be flashing continuously.

Activate the Electrical System &Accessories

If you can get in the car, or you can disable the alarm, but the immobilizer has the engine locked out, you might be able to disengage it with the right key.

To do this, you want to insert the primary key for the vehicle (Not the Valet Key). Then, Turn on the ignition to the point that just the accessories activate without cranking the engine.

Re-check the Anti-Theft System without Cranking the Engine

Check the anti-theft light to see if it stopped blinking when you activated the accessories. It recognizes your key, and the alarm portion of the system will usually disengage. However, the immobilizer chip in the ECU is often slow to get this message.

Don’t give in to the temptation to crank the engine. Instead, turn the key to the OFF position and let the car sit for at least five minutes. This will allow the system and the immobilizer chip to coordinate and restart the system as normal.

Try to Start the Engine

After waiting 5 minutes, insert the key into the ignition and turn it to crank the engine. If it clicks and sits inert, the immobilizer chip still locks out the engine. If it cranks for a long time but still won’t start, the immobilizer chip is still preventing the ECU from initiating the internal combustion process.

Don’t be shocked if your first attempt at this reset doesn’t work. You might have to go through the process three times before the immobilizer chip lets the engine turn over.

Disengaging an Anti-Theft System on a Keyless Entry Vehicle

Keyless entry car theft NO MORE how to totally protect your car from keyless theft

If you have a keyless entry feature on your car, and someone attempts to break in, activating the anti-theft system, the keyless entry will also be locked out. If your car has a lock and a keypad, you usually need to insert the primary (Not Valet) key and the following steps.

  • Step One: Insert the key into the lock and hold it there for 5 to 10 seconds to let the anti-theft system detect the RF signal in the key fob.
  • Step Two: Enter the correct entry code.
  • Step Three: Attempt to unlock the car door with the key physically but hold it in the unlocked position for 30 seconds without actually trying to open the door handle.

This will tell the anti-theft system that you have the correct key, and it will disengage the immobilizer chip.

  • Step Four: Use the door handle to open the door.
  • Step Five: Once you’re in the car, immediately try to start the engine with the primary key.

Some anti-theft systems have a minimum threshold of time between disengaging and opening the door and starting the engine. If you wait to turn the key for too long, the system might rearm itself, thinking a would-be thief is trying to hot wire the ignition.

Disengaging the Anti-Theft System with the Key

Some models allow you to reset the anti-theft system with the physical key. Here again, you need to be working with the primary key and not a valet or backup spare key.

  • Step One: Use the key fob to turn off the alarm.
  • Step Two: Insert the key in the driver’s side door lock.
  • Step three: Physically unlock and lock it three times.

Disengaging a Steering Wheel Lock

Disabling Steering Wheel Lock - RX300

Some vehicle anti-theft systems are designed to lock the steering wheel and prevent the key from turning in the ignition. If someone attempted to get into your car, or the anti-theft system was set off accidentally, you might be able to unlock the steering wheel. However, you’ll need the primary key to do so.

  • Step One: Insert the primary key into the ignition without turning it.
  • Step Two: Try turning the wheel in the opposite direction.
  • Step Three: With pressure still on the wheel, try to turn the key forward.
  • Step Four: If the key doesn’t budge, try to turn it back to the setting behind the OFF position.
  • Step Five: Jiggle the key in the ignition to help make sure all the tumblers align.
  • Step Six: Repeat steps two and three

It might take multiple attempts to get the steering wheel and ignition cylinder to unlock especially if you have an older, worn-out key, as the teeth won’t always synch perfectly with the tumblers inside the cylinder.

Resetting the Immobilizer Chip

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If all your attempts to disarm the anti-theft system with the key and key fob have failed, then your last, best hope is to try to reset the immobilizer chip. Sometimes, the immobilizer chip is built into the steering column, and you’ll have to take off panels and trim pieces to get at it.

You can check your owner’s manual for the location, and it might include specs for how to reset it.

Frequently Asked Questions

Can I change the setting of my shock sensors myself?

If overly sensitive shock sensors keep triggering your car’s anti-theft system, you might be able to adjust the sensitivity yourself. However, this isn’t always easy. With some models, the wise move is to bring it to a dealership. If your car is still under warranty, alarm and anti-theft system problems are usually included in the warranty coverage.

Can Disconnecting the Battery Reset the Anti-Theft System?

In some older vehicles with first-generation anti-theft systems, disconnecting the battery and waiting 10 minutes or so was sufficient for disabling the anti-theft system. Of course, car thieves were keen on this, and many modern systems now have physical lockouts on the steering wheel or other features like the immobilizer chip that will stay engaged even if the battery is removed and power is later engaged.

Disconnecting the battery is certainly worth a shot. Just don’t be surprised if other anti-theft features are blocking you from starting or driving the car.


If your anti-theft system was accidentally engaged with the car alarm or due to an overly sensitive shock sensor, then you should be able to start the car again with the key fob. If the fob battery is dead, or you have the valet key, you’ll need to replace the battery and make sure you’re using the primary key.

If your car’s battery or alternator is dying, it will engage the anti-theft system as a way of alerting you to the problem. Cleaning the battery terminals and/or replacing the battery and alternator might be in order before you can start the car again.

If your anti-theft system was triggered by someone else trying to break into your car, you’ll have to take steps to reset the system completely. This usually starts with using the primary key and fob to unlock the door. In some models, you might have to use the key physically. You might also have to get the steering wheel to unlock enough to let you turn the key in the ignition.

Sometimes, the immobilizer chip engages and won’t disengage. In a scenario like this, you’ll have to find the reset method specific to your make and model, or you’ll have to get the car towed to a certified dealership.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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