How Does a Remote Keyless Entry System Work in a Car?

by Peter Boysen

There are two main kinds of remote keyless entry devices: the controller you hang from your visor that opens and closes your garage door, and the black fob you put on your key ring that locks and unlocks your car door, arms and disarms your car alarm, and maybe even opens the trunk. When you push one of the buttons, a transmitter is activated, and it sends out a code to the receiver. This receiver is inside the car (or the garage), and works much like the ones you find in radio-controlled toys.

Everyone who has a keyless entry system needs a unique code, at least within a large geographic area. People prowl with radio scanners, too, and could capture your code that way. Once they have your code, all they need to do is resend it, and your car (or garage) will open for them, too. Your controller uses what is known as a rolling code, or a hopping code. Commonly, systems use a 40-bit rolling code, which creates around a trillion possible codes.

The innards of your remote fob

The controller chip's memory holds your 40-bit code. Pushing the button on your transmitter sends two codes: the 40-bit code and a function code. This function code sends instructions for a specific action on the receiving end. Use it to lock your car, open the trunk or to perform other tasks. The receiver controller chip has the same 40-bit code. When the codes match, the receiver performs the assigned function. Nothing will happen if you don't achieve a match. The receiver and transmitter also have a "pseudo-random number generator" which picks the code for the next task.

If you accidentally hit the button, but you're too far away for your door to open, or your car to unlock, the receiver will also take the next 256 codes that would have happened. However, if this doesn't work, turn the ignition on and off 8 times in less than 10 seconds. Leave the car on the eighth time. This prompts your security system in the car to go into programming mode. Push a button on all of the fobs that you want to use with the car. Then, switch the car off.

About the Author

Teacher, freelancer, marathoner in the Dallas area.

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