(August 2012) (Learn how and when to remove this template message) A transponder chip disarms a vehicle immobiliser when the car key is used to start the engine. The majority of keys to cars built after 1995 contain transponder chips. When the car key is turned in the ignition, the engine control unit (ECU) on the car sends an electronic message to the key, and it will allow the car to start only provided it receives the correct message back.