immobiliser or immobilizer is an electronic security device fitted to an automobile that prevents the engine from running unless the correct transponder car key (or other token) is present. This prevents the car from being "hot wired" after entry has been achieved and thus reduces motor vehicle theft. Research shows that the uniform application of immobilisers reduced the rate of car theft by 40% from 1995 to 2008.
Early models used a static code in the ignition key (or key fob) which was recognised by an RFID loop around the lock barrel and checked against the vehicle's Engine Control Unit (ECU) for a match. If the code is unrecognised, the ECU will not allow fuel to flow and ignition to take place. Later models use rolling codes or advanced cryptography to defeat copying of the code from the key or ECU.
The microcircuit inside the key is activated by a small electromagnetic field which induces current to flow inside the key body, which in turn broadcasts a unique binary code which is read by the automobile's ECU. When the ECU determines that the coded key is both current and valid, the ECU activates the fuel-injection sequence.