What Does an Engine Immobilizer Do?

by Robert MooreUpdated July 11, 2023

Prior to 1984, automotive anti-theft systems or immobilizer systems were widely unregulated. The standard theft-deterrent practices followed by manufacturers consisted of nothing more than a few stamps of the vehicle's identification or serial number and uniquely cut door and ignition keys as a security system. Enter the ‌Motor Vehicle Theft Enforcement Act of 1984‌, aka Chapter 331 of Title 49 of the United States Code. This law went into effect for the 1987 model year and required manufacturers to stamp or etch the vehicle and major replacement parts with the Vehicle Identification Number, or VIN. By law, 12 stamps were required on two-door vehicles and 14 stamps on four-door vehicles. On Sept. 8, 1987, the National Highway Traffic Safety Administration amended the law to allow manufacturers to request an exemption from the minimum number of stampings for a specific vehicle line. The stipulation, however, was that the manufacturer had to include an anti-theft device that was determined to be just as effective as VIN stamping. That is how the seed was planted for the engine immobilizer and its widespread use from manufacturer to manufacturer to prevent car theft and improve car security as a whole when cars are driver away from the dealership.

Different Types of Immobilizers

Immobilizers come in two different flavors: passive and non-passive. Passive immobilizers are found in every production car manufactured today in the United States and work automatically without any input from you to protect your car from car thieves. That’s right: Bet you didn’t know your car’s immobilizer system works even if you don’t lock your doors and set the alarm. Non-passive systems are commonly found on aftermarket alarms systems or are seen in the form of a hidden cutoff switch in modern cars. If you have a non-passive engine immobilizer, it only functions if you set the alarm or activate the hidden switch on the car key or on the engine control unit. This is your classic car alarm that works as an electronic security device. These two types of security codes offer two-tier security for the ignition circuit and is made by car manufacturers to ensure vehicle immobilization for new cars if there is any kind of theft attempted.

Basic Functionality

On vehicles that come equipped with passive engine immobilizers, the whole system is built into the computerized controls of the vehicle. Depending on your make and model, this engine immobilizer system can include the powertrain control module, body control module, engine start, keyless entry, steering wheel lock, instrument cluster, radio control system, door-lock control system, ignition system, fuel system and the ignition switch. If the immobilizer is triggered, it may prevent the fuel injectors or ignition coils from firing, the fuel pump from turning on, the starter from engaging or a combination of these preventative measures. If they can’t start it, they can’t drive it because the coded key won’t let them.

So How Does the Car Know It Is Me Trying to Start the Engine?

Passive immobilizers have advanced over the years. Early systems passed a low-voltage signal into the key, which was equipped with a resistor, and then back to the computer. The computer was programmed to expect a certain voltage drop in that signal, and if it didn’t get the proper return voltage -- you guessed it -- the engine wouldn’t start. Other systems had a sensor inside the ignition lock cylinder that would complete the aforementioned circuit when the cylinder was turned to the “On” position. Nowadays, systems use transmitter chips that are built into the ignition key or the smart key fob if your vehicle is equipped with push-to-start functionality. On this type of system, when you attempt to start the engine, a radio signal is sent from the key or fob to the receiver in the vehicle. This receiver then tells the BCM or PCM that the correct key was used, and that module allows the engine to start. Any time the system detects trouble -- even if a part of the system fails -- the system prevents the engine from starting. So remember, it doesn’t mean your crazy ex-boyfriend or that hoodlum teenager down the road tried to steal your car.

The Immobilizer Bypass Myth

Bypassing a manufacturer's stock engine immobilizer has to be one of the most common Internet searches related to alarm systems. Sometimes immobilizers can seem more of a hassle than a blessing when problems with the system pop up just as you have somewhere important you need to be. There are well-known ways to bypass older systems, and once that knowledge became widely available, engineers had to find new ways to protect your vehicle.


There are special modules available to allow aftermarket remote starter systems to bypass the engine immobilizer. These normally require you to install a second key inside the remote start module.

The truth is, it is practically impossible to bypass most current manufacturer-supplied immobilizers. You can’t just cut out the control module or cut some wire. Because your car’s immobilizer is built into the vehicle's engine and body control systems, you’re stuck with a non-running vehicle until your local mechanic can sort out the problem and eradicate any gremlin causing a malfunction.

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