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Immobilizers are electronic security devices installed on cars that prevent the engine from starting unless the correct transponder key is present. It stops the car from being "hot wired" after entry of the car and thus reduces motor vehicle theft. It contains a microcircuit located inside the keys which are activated by a magnetic field that induces a current flow to go through inside the key body, This broadcasts a unique binary code which is read by the automobile's electronic control unit. (ECU). Once the ECU determines that the coded key is both current and valid, the ECU will now activate a fuel-injection sequence.

For some vehicles, attempts to use an authorized key sequence will enable the vehicle to activate a timed no-start condition and in some highly advanced systems, it may even use satellite or mobile phone communication to provide a security alert that an authorized attempt was made to code a key. Some variants of Immobilizer systems tend to remember the last key code even for so long, it may even accept a nontransponder key even after the original key has been removed from the ignition for a few minutes,