Transponder keys are factory-made accessories that come with a car and provide for radio transmission between the car and the key. This radio transmission allows for a key to be programmed to only start a specific, individual car. Thus, only the person who is in possession of a car’s transponder key will be able to start and drive it. Transponder keys reduce the opportunity for car thieves to steal a car.

A transponder key blank, which has not been programmed, can crank a car. It can take more than 30 minutes for the car key transponder to align itself with the system of the car, as the electronics within the car reset. Transponder car keys are available for many different cars, but are more common in high-end models.

The way transponder key programming works involves a microchip that resides in the top of the key. This microchip has a serial number, which a transponder key programmer has made unique to that key. The electronic/engine control units (ECU) will send a request to the vehicle for it to validate the serial number. If the car doesn’t recognize any serial number within its set of confirmed and correct numbers, the car will not start.

When buying a vehicle that includes transponder keys, it's a good idea to have at least two keys. Sometimes keys are lost, so having a second valid, programmed transponder key is important. If replacement transponder keys are needed, they can usually be made by a vehicle dealer or locksmith. It may be possible for a driver to program his or her own blank keys, but the process can vary by car.

The technology involved in programming keys to specific cars has lead many vehicle manufacturers and insurance companies to consider cars with transponder keys to be virtually theft-proof. There are many reports, however, of thieves finding ways around the transponder, sometimes simply by towing the car away. Some manufacturers have been resistant to releasing their programming codes to locksmiths, possibly for fear of theft, but most information is available to authorized users. The perception that transponder keys make a car nearly impossible to steal has caused problems for some owners, as it can be more difficult to convince an insurance company when a theft does take place.