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How does a car key fob work?

TechnologyLesson on requestFree to read

Quick answer

A key fob is a pocket radio transmitter. Pressing a button sends a short, encrypted signal with a one-time rolling code to a receiver in the car. The car checks the code against its own counter and secret key, and if it matches, it unlocks the doors, locks them or opens the boot.

A car key fob is a small battery-powered radio transmitter that sends a coded signal to a receiver in the car when you press a button. Each press uses a new rolling code generated from a shared secret, so the car accepts it but rejects a recording of an old signal.

Step by step

  1. You press the unlock button, which powers the fob's chip from its small coin-cell battery.
  2. The chip advances its internal counter and encrypts it with a secret key to create a fresh rolling code.
  3. The fob broadcasts the code along with its ID and the unlock command as a brief radio signal.
  4. The car's receiver decrypts the code and checks that its counter value is within the expected range ahead.
  5. When the check passes, the car updates its own counter and powers the door lock actuators to release them.

Key things to understand

  1. Pressing a button wakes a tiny chip in the fob that builds a digital message containing an ID, a command and a code.
  2. The fob transmits that message as a short radio burst on a licence-free band, commonly 315 MHz or 433 MHz.
  3. A receiver in the car checks the fob's ID and decrypts the code using a secret key stored in both devices.
  4. A counter inside each device advances with every press, so a valid code is never reused and replayed signals are refused.
  5. If the code checks out, the body control unit drives the door lock motors, flashes the indicators or opens the boot.

Labelled diagram

  1. Fob button

    Wakes the chip and selects the lock, unlock or boot command.

  2. Rolling code chip

    Encrypts an advancing counter with a secret key to make a one-time code.

  3. Radio signal

    A short burst on a licence-free band carries the code to the car.

  4. Car receiver

    Decrypts the code and checks the counter against its own record.

  5. Lock actuators

    Small motors in each door release the latches once the code is accepted.

From button press to unlocked door

Real-world examples

  • Many cars sold in India and Europe use fobs that transmit around 433 MHz, while North American models often use 315 MHz.
  • A driver in a crowded mall car park presses the fob's lock button and the car flashes its indicators to confirm.
  • Push-button-start cars read a transponder chip in the fob before the engine management system allows the engine to run.
  • Some scooters sold in India now use smart keys that unlock the steering and ignition when the fob is nearby.

Common mistake

✗ Myth: A key fob sends the same fixed code every time, like a garage remote from decades ago.

✓ Fact: Modern fobs use rolling codes, so each press produces a new code that the car accepts only once. Capturing and replaying a used code does not unlock the car, although relay attacks on passive systems remain a separate risk.

People also ask

How does a car key fob communicate with the car?
It uses radio waves. The fob contains a small transmitter and antenna that send a coded digital message on a licence-free frequency when a button is pressed. A receiver module in the car picks up the signal, verifies the code with its stored secret key and then tells the locks what to do.
Why is my key fob not working?
The most common cause is a weak coin-cell battery, which shows up as shrinking range before it fails completely. Other causes include strong radio interference nearby, a fob that has lost synchronisation with the car after many presses out of range, physical damage or a fault in the car's receiver.
How far does a car key fob work?
Button-press fobs typically work from several metres up to a few tens of metres, depending on the design, battery level and obstacles. Passive keyless entry is deliberately short-range, usually responding only within a metre or two of the car, so it unlocks for someone standing at the door.
How does keyless entry work without pressing a button?
Passive keyless systems use low-frequency antennas in the car that send a short-range wake-up signal. A fob within about a metre or two replies on its radio frequency, and after the challenge and response match, the doors unlock when you touch the handle.
Can I start the car if the fob battery is dead?
Usually yes. The immobiliser uses a passive transponder chip powered by the car's own antenna coil, so holding the fob against the start button or a marked spot lets the car read it. Most fobs also hide a mechanical key blade for the door.
What is a relay attack on a car key?
Thieves with two radio devices capture the signal from a passive fob inside a house and relay it to the car outside, fooling it into unlocking and starting. Storing the fob in a signal-blocking pouch or disabling passive entry reduces this risk.
What battery does a car key fob use?
Most fobs use a flat lithium coin cell, often a CR2032 or similar size printed inside the case. It typically lasts a few years, and shrinking range is the usual sign that it needs replacing.

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