Passive keyless entry (PKE) is a vehicle access system that unlocks your car automatically when you walk up to it with the key fob in your pocket or bag. Unlike older remote systems where you press a button on a fob, PKE requires no interaction at all. The car detects that you’re nearby and unlocks when you touch or approach the door handle.
How It Differs From Remote Keyless Entry
The distinction between passive and remote keyless entry comes down to one thing: whether you press a button. Remote keyless entry (RKE) requires you to physically press a button on your fob to send a signal to the car. Passive keyless entry does everything based on proximity. You walk toward the vehicle, and it recognizes you’re there. You touch the door handle, and it unlocks. You walk away, and it locks behind you.
Both systems use encrypted radio signals, but the communication pattern is fundamentally different. With RKE, you initiate the conversation. With PKE, the car initiates it by constantly polling for your key fob when you’re within range.
The Two-Frequency Handshake
PKE systems rely on a two-way conversation between the car and your key fob, and that conversation happens on two different radio frequencies. The car broadcasts a low-frequency (LF) signal at 125 kHz from antennas placed around the vehicle, typically in the door handles, trunk, and cabin. This LF signal has a short range by design, usually reaching only a few feet. Its job is simple: wake up any nearby key fob and ask, “Are you the right key?”
When your fob picks up that LF wake-up signal, it responds on a separate ultra-high-frequency (UHF) channel at 433 MHz (or 315 MHz in North America). This response carries an encrypted code that the car’s receiver checks against its stored credentials. If the codes match, the car unlocks. The entire exchange takes a fraction of a second.
The reason for using two frequencies is practical. The low-frequency signal is easy to contain within a small area, so the car can tell roughly how close you are. The higher-frequency response from the fob travels farther and carries data more efficiently. Together, they create a system that’s both spatially aware and fast.
What Happens When You Start the Car
PKE systems typically extend beyond door locks to include push-button ignition. Once you’re inside the vehicle, a separate set of interior LF antennas confirm that the fob is actually in the cabin, not just near the outside of the car. This prevents the engine from starting if someone touches your door handle while your fob happens to be within range but you’re not actually in the vehicle. Only after the system confirms the fob is inside does the push-button start activate.
Encryption and Security
Modern PKE systems encrypt the radio conversation between fob and car using AES-128, the same encryption standard used in banking and government communications. Each time the fob and car communicate, they use a rolling code, meaning the encrypted signal changes with every interaction. Even if someone intercepted one signal, it would be useless for a future unlock attempt.
This is a significant upgrade from older systems. Earlier remote keyless entry systems used a code-hopping scheme called KeeLoq, which researchers demonstrated could be broken through power-analysis attacks as far back as 2008. AES-128 is far more resistant to these kinds of exploits.
That said, PKE systems do have a well-known vulnerability: relay attacks. Two thieves working together can use signal amplifiers to stretch the LF wake-up signal from your car to wherever your fob is, even inside your house. Your fob thinks it’s next to the car, responds normally, and the car unlocks. This doesn’t break the encryption. It tricks the system into thinking you’re standing next to the vehicle when you’re not.
How Digital Keys Are Changing the System
The next evolution of passive keyless entry replaces the physical fob with your smartphone. The Car Connectivity Consortium, an industry group that includes most major automakers, released its Digital Key 3.0 specification in May 2021. This standard uses ultra-wideband (UWB) radio technology alongside Bluetooth to let your phone function as a car key.
The process starts with Bluetooth establishing a trusted connection between your phone and the car. Once that pairing is confirmed, UWB ranging kicks in to measure the precise distance between the two devices. UWB operates on frequencies around 7.7 to 8.2 GHz and transmits at extremely low power, more than 1,000 times lower than a typical Wi-Fi signal. It only uses the radio spectrum for brief bursts, occupying the airwaves about 1 to 5 percent of the time during a ranging session.
The key advantage of UWB over traditional PKE is spatial precision. UWB can measure distance down to centimeters, which makes relay attacks far more difficult. The system knows not just that your phone is “nearby” but exactly how far away it is and in which direction. If someone tries to relay the signal, the distance measurements won’t add up, and the system rejects the unlock attempt. This is built on the IEEE 802.15.4z standard, which was specifically designed with secure ranging in mind.
Fob Battery Life
Because PKE fobs are constantly listening for the car’s wake-up signal rather than sitting completely idle, battery life is a common concern. Most fobs use a CR2032 coin cell battery, a small, flat lithium battery about the size of a nickel. In a car key fob, a CR2032 typically lasts 2 to 4 years, though some can stretch to 4 or 5 years since the fob only transmits for brief moments a few times per day.
The drain comes from those short, high-current bursts of radio transmission rather than from a constant power draw. How often you use the car, temperature extremes, and whether the fob sits near other electronic devices all affect battery life. Most vehicles will display a warning on the dashboard or instrument cluster when fob battery power is getting low. If the battery dies completely, most PKE-equipped cars have a backup method: a physical key blade hidden inside the fob and a spot on the steering column or center console where you can hold the dead fob close enough for a very short-range signal to start the car.
Protecting Against Relay Attacks
Until UWB-based digital keys become standard across the industry, the relay attack vulnerability in traditional PKE remains real. A few practical steps reduce the risk. Storing your fob in a signal-blocking pouch (sometimes called a Faraday pouch) prevents it from receiving the car’s LF signal when you’re not using it. Some fobs have a motion sensor that puts the fob to sleep after being stationary for a few minutes, so a fob sitting on your nightstand won’t respond to a relayed signal. Certain automakers also let you disable the passive entry feature entirely through the vehicle’s settings, reverting to button-press unlocking when you want the extra security.