What to Do If Your Car’s Anti-Theft System Prevents It from Starting

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Diagnose the problem and start your car correctly

Co-authored by Eric McClure Reviewed by Jason Shackelford

Last Updated: October 31, 2024 Fact Checked

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• Why won't my car start?
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• Using Your Spare Key
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• Unlocking the Door
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• Resetting the Battery
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• Rebooting the ATS
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• What if my car still won’t start?
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• Can I bypass the ATS?

|Show more |Show less

If your vehicle won’t turn on and you see a little car and lock light on your dashboard, your anti-theft system is probably preventing the car from turning on. As confounding as this may be, the solutions here are probably very easy to carry out, and you don’t need any fancy tools or complex diagnostic gear. In this article, we’ll show you how to fix the anti-theft system so that you can back on the road.

Things You Should Know

• Try using your spare key. If the vehicle turns on, the transponder in the other key is dead. Take it to a dealership to get it fixed.
• Unlock your door and disconnect your battery to reset the vehicle’s computer and turn the anti-theft system off.
• Manually reset the transponder connection by turning the key to the “on” position for 10 minutes, then turn it "off" for 30 seconds. Repeat the process 3 times.

Steps

Section 1 of 7:

Why won’t the anti-theft system let my car start?

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1. Your vehicle isn’t recognizing your key or fob. Most modern vehicles have a sensor inside of the key (or fob) that basically tells your vehicle, “This is the correct key and you can start the vehicle.” If that communication between your key and the vehicle’s computer doesn’t take place, your vehicle’s anti-theft system will kick on and prevent you from starting the car.

   • The anti-theft system is designed to keep your car from turning on if someone tries to hotwire it or turn the ignition cylinder with a screwdriver.
   • You can tell your anti-theft system is keeping the car from turning on if there’s a car and lock or car and key light popping on your dashboard. It may blink depending on your model.
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Section 2 of 7:

Solution #1: Try Your Spare Key

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1. If your key is malfunctioning, the spare key will work. If your original key isn’t sending the correct signal to your vehicle, the engine won’t turn on. If you’re lucky enough to have a spare key laying around somewhere, grab it and see if you can start your car. If you can, the other key wasn’t working correctly.

   • Take your old busted key to any dealership that sells your vehicle’s make. They’ll reprogram and fix the key for you.
   • If the vehicle doesn’t turn on with your spare key, the problem is unlikely your keys. It’s much more likely that your vehicle’s computer is at fault.

Section 3 of 7:

Solution #2: Unlock Your Door

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1. If you have an old-school key, lock your car and then reopen it. Some anti-lock systems turn on if the door isn’t opened using the appropriate key. This is designed to prevent thieves from breaking in and using a key you left inside. But if the door was never closed correctly or there’s a miscommunication with the key, your vehicle will think it’s a theft. Turn the car off, close the door, and use your key to open the door.

   • If the ignition works as intended, you’ve successfully fixed the problem.
   • Just FYI, this is also how you turn off a car alarm that won’t stop.
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Section 4 of 7:

Solution #3: Reset Your Car Battery

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1. If your vehicle’s computer is buggy, resetting the battery will fix things. With your vehicle off, pop the hood and disconnect the negative battery terminal. Then, turn on some interior lights and wait for them to completely go out to fully discharge the battery. Reconnect the terminal and try turning your car on. If it works, your vehicle’s computer was just malfunctioning.

   • If your vehicle won’t respond once you reconnect the battery, your battery is probably bad. This could definitely cause the original anti-theft error, so replace your battery to see if that helps.

Section 5 of 7:

Solution #4: Reset the Anti-Theft System

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1. Manually reset the anti-theft system by turning your car on and off. Insert your key and hold it in the “off” position for 30 seconds. Then, turn the key to “on” and wait at least 10 minutes. Flip the position back to off for 30 seconds and repeat the cycle. Do this for 3 cycles (a total of 30 minutes) and then try turning your vehicle on.

   • If your car starts, the anti-theft system just needed to be reset. This off-on repetition resets the connection between the key and the computer, so if it’s a basic bug, this should do the trick.
   • On some makes and models, you only need to do this for 10 minutes, not 30.
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Section 6 of 7:

What if my car still won’t start?

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1. See a mechanic to have your vehicle inspected and repaired. If you still can’t get the anti-theft system to turn off, there's likely something wrong with your vehicle’s engine control module, which is the central computer in your vehicle. A mechanic should be able to sort out the problem.

   • If you have a push-to-start vehicle, there could be something wrong with the primary ignition system.

Section 7 of 7:

Can I bypass my vehicle’s anti-theft system?

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1. In most cases, no, you cannot trick this system. The anti-theft system of most vehicles is designed to not be easily disabled. If it were super easy to bypass the anti-theft system, it’d be way too easy for criminals to break in and take your car for a joy ride.

   • The anti-theft system is a complicated bit of software; it’s not like there’s a physical switch you can flip to turn the anti-theft system off.
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Tips

• Some vehicles have a dedicated fuse for the anti-theft system. If this fuse is blown, your anti-theft system may assume someone is tampering with the vehicle and it won’t start. Purchase a replacement fuse and find the fuse location on your electrical panel by reading your manual. Remove the old fuse and put the new fuse in place.
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• Bad wiring can also cause an anti-theft system to turn on when it shouldn’t, although this seems to be a relatively rare cause on most vehicles.
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