# Immobiliser: what it is, how it works and do you need an add-on _Updated: 8 November 2026_ **Short answer:** An immobiliser is an electronic lock that stops the engine from starting without the right key. Almost every newer car has one from the factory, but thieves trick it. An extra immobiliser also asks for your fob or code, so the engine will not start even when the thief has a key. > **The service we recommend: Immobiliser** The engine will not start without your fob or code, even with the key. We tell you the price when you tell us the model. [More about the service](https://autoindustrija.rs/en/services/immobiliser) ## The factory immobiliser checks the chip in the key There is a small chip in the key. When you turn the key or press the start button, the car asks the chip who it is. If the answer is not the one the car knows, the engine will not start, even though the dashboard has lit up. The chip has no battery. The signal from the antenna at the lock powers it. Newer cars ask for a different code at every start, so a code copied from the key will not start the engine. The immobiliser switches on by itself as soon as you turn the engine off. While it is on, the engine does not get fuel, ignition or current for the starter motor. So the car will not start even if a thief joins the wires under the steering wheel. The immobiliser works. When it became mandatory in every new car in the European Union, it cut car theft by about 40%, a study showed. So thieves started to fool the immobiliser itself. ## Thieves fool it in 3 ways The factory immobiliser only checks whether the key is known. So the thief tricks the car into believing the key is genuine: - **A new key through the OBD port:** The OBD port is the spot under the steering wheel where diagnostics is plugged in. The thief plugs his own device into it and the car learns a new key. From then on the immobiliser treats that key as genuine. [More about theft through OBD](https://autoindustrija.rs/en/advice/obd-car-theft) - **Keyless theft:** A keyless key unlocks and starts the car from your pocket, because the car asks the key by radio on its own. Thieves pick up your key signal and relay it to the car. The immobiliser gets a correct answer, because the key really is yours. [More about keyless theft](https://autoindustrija.rs/en/advice/keyless-car-theft) - **The headlight attack:** The thief pulls away part of the bumper, connects to the wiring at the headlight and sends the car a fake message that the key is genuine. This is how the Toyota RAV4 and Lexus were stolen, and models of other makes can be stolen the same way. [More about the headlight attack](https://autoindustrija.rs/en/advice/headlight-can-injection-theft) In all 3 cases the car thinks it has the right key, so the factory immobiliser lets the engine start. What stops the thief is a second condition he does not have: your fob or code. ## An add-on immobiliser also needs your fob or code An add-on immobiliser is a separate small unit that works with the factory one. The engine starts only when the car has both the right key and your confirmation. The confirmation can be a fob, a code or a phone: - **Fob:** You carry a small fob in your pocket. When you get into the car, the immobiliser recognises it by itself and the engine starts as before. You do not press anything. - **Code:** The code is a sequence of presses on buttons the car already has, on the steering wheel or the door, for example the window switch. You choose the sequence. There is nothing to lose or steal, but you enter the code on every drive. - **Phone:** With some units the phone works as the fob. You carry the phone, and the immobiliser recognises it when you switch on the ignition. When the phone is not in the car, the engine will not start. Do not keep the fob with the car key and do not leave it in the car. While the fob is in the car, the immobiliser does not protect it. If a thief takes the key together with the fob, he has everything. ## An immobiliser protects only the engine start - It does not stop a thief from making a new key through the OBD port. It only stops the engine from starting with that key. Making the key is stopped by [OBD port relocation](https://autoindustrija.rs/en/obd-port-relocation). - It does not prevent a break-in and theft of things from the cabin. The siren and a message to your phone come from an alarm. [More about the alarm](https://autoindustrija.rs/en/services/car-alarm-installation) - It does not show where the car is if the thief takes it on a tow truck. That is what GPS is for. [More about GPS tracking](https://autoindustrija.rs/en/services/gps-vehicle-tracking) The best protection is a combination, because each protection stops a different way of stealing. [More about the best theft protection](https://autoindustrija.rs/en/advice/best-car-theft-protection) ## Who needs an add-on immobiliser **Whether you need an extra immobiliser depends on your car** - **A thief can get to your key:** thieves steal keys from homes, bags and jackets. With an add-on immobiliser the engine will not start even with your key, unless the thief also has the fob or code. - **You have a keyless key:** a pouch does not let the key signal out, and the engine will not start without your fob even if the thief relays the signal. [More about the key pouch](https://autoindustrija.rs/en/advice/faraday-pouch-car-key) - **You drive a Toyota or Lexus:** thieves steal these makes through the wiring at the headlight as well, where OBD protection does not help. An immobiliser does. [How to protect a Toyota or Lexus](https://autoindustrija.rs/en/brands/toyota) - **You have an ordinary key:** OBD port relocation stops the thief from making a new key, and the immobiliser stops the engine starting even with a key. - **You already have an alarm:** the alarm makes noise and alerts you, while the immobiliser stops the engine from starting. Some insurers give a discount on comprehensive cover if the car has an extra anti-theft device. [More about comprehensive insurance and theft](https://autoindustrija.rs/en/advice/comprehensive-insurance-car-theft) ## When the immobiliser will not let you start either Sometimes the factory immobiliser does not recognise even the right key. Then the dashboard lights up, the engine does not start, and the light with a key or a car with a key stays on or flashes. [More about the immobiliser light](https://autoindustrija.rs/en/advice/immobiliser-warning-light) If the chip in the key is damaged, the engine will not start until a new key is programmed into the car. Diagnostics reads the immobiliser faults and shows where the fault is. [More about a car that will not start because of the immobiliser](https://autoindustrija.rs/en/advice/car-wont-start-immobiliser) ## Frequently asked questions ### Does my car already have an immobiliser? Almost certainly, if it was made after 1998. For an older car, the owner’s manual says so. If it has none, we fit an add-on immobiliser to an older car too. ### Does a car in Serbia have to have an immobiliser? The Serbian rulebook on motor vehicle categories requires every passenger car to have a device against unauthorised use. It can be a steering lock, a gear lever lock or a system that stops the engine from starting, such as an immobiliser. ### Can an add-on immobiliser shut the engine off while driving? It does not shut the engine off on its own. The immobiliser checks the fob when you start the car and pull away. Some devices also have protection for when someone takes the car from you by force. They first warn with a sound, and only then stop the engine. ### Does an immobiliser drain the battery? It uses very little current while the car is parked. ### What does immobilizer mean? Immobilizer is the English name for an immobiliser. Car owner’s manuals use it too. Have a question about your car? Message us on WhatsApp or call 067 774 1606. ## Read also - [Car theft in Serbia: 9 ways thieves steal a car](https://autoindustrija.rs/en/advice/how-cars-are-stolen-in-serbia) - [OBD car theft: how thieves make a new key](https://autoindustrija.rs/en/advice/obd-car-theft) - [Keyless theft (relay attack): how it works, how to stop it](https://autoindustrija.rs/en/advice/keyless-car-theft) --- Page on the website: https://autoindustrija.rs/en/advice/what-is-an-immobiliser auto-industrija, Belgrade. Phone and WhatsApp: +381677741606. E-mail: kontakt@autoindustrija.rs.