How a Car Key Unlocks a Car

A car key unlocks a car either by mechanically turning a lock to move internal linkages, or by sending an authenticated electronic signal—via RFID, radio, Bluetooth, NFC, or ultra‑wideband—that tells the vehicle’s control module to release the door locks; most modern systems also authenticate the key to permit engine start. This article explains how mechanical, remote, and “smart” keys work, what happens behind the scenes when you press unlock or touch the handle, and how security features protect against theft.

The main ways car keys unlock doors

Mechanical blade keys

Older and base-model vehicles use a cut metal blade that physically aligns pins or wafers in a door lock cylinder. Turning the key rotates a cam that moves rods or an electric actuator to retract the latch. This works without power and cannot be hacked remotely, but it offers no electronic authentication by itself.

Remote keyless entry (RKE) fobs with buttons

Pressing “unlock” on the fob transmits a short encrypted radio burst to the car. The body control module (BCM) verifies the rolling code or challenge-response and, if valid, energizes door-lock actuators. Typical radio bands include 315 MHz (North America), 433.92 MHz or 868 MHz (Europe), and 902–928 MHz (US). Fobs also usually carry a hidden mechanical blade for emergencies.

Passive keyless entry (PKE) and “smart” keys

With hands-free systems, the car senses a valid key nearby when you touch the handle. Low-frequency (circa 125 kHz) antennas in the door handles and cabin “ping” the key, which replies over RF (sub‑GHz) or Bluetooth Low Energy (2.4 GHz). If proximity and cryptography check out, the BCM unlocks. Many newer models add ultra‑wideband (UWB, typically 6–8.5 GHz) ranging to measure distance precisely and block relay attacks.

Phone, card, and NFC keys

Some vehicles support Bluetooth “phone-as-key,” digital keys stored in Apple Wallet/Google Wallet (NFC/BLE/UWB), or credit-card-style keys. For backup, tapping the phone or card on a marked NFC area can unlock and authorize start even with a dead phone battery (using passive NFC).

What components are involved in electronic unlocking?

Several modules collaborate each time you unlock electronically. The following list outlines the main parts you interact with indirectly every time you use a modern key system.

• Key fob or smartphone: Stores cryptographic secrets; transmits responses and commands.
• Low-frequency antennas: Located in door handles and cabin; wake and localize the key for passive entry/start.
• RF/BLE/UWB receivers: In the car; receive the fob’s radio messages.
• Body Control Module (BCM): Verifies credentials; commands door-lock actuators and alarms.
• Door-lock actuators: Electric motors/solenoids that physically lock/unlock latches.
• Immobilizer/ECU: Separately authorizes engine start via a cryptographic handshake.

Together these components ensure that only an authenticated, nearby key can conveniently unlock doors and enable starting, while keeping the mechanical system available as a fail-safe.

What happens when you press “unlock” on a fob?

Although it feels instantaneous, several steps occur in milliseconds whenever you use a button-based remote to open your car. This sequence summarizes the typical process.

1. You press the fob button; its microcontroller wakes and assembles a message with a rolling code or cryptographic response.
2. The fob transmits a brief radio signal at the region’s allowed frequency (e.g., 315/433/868/915 MHz).
3. The car’s receiver forwards the packet to the BCM, which checks the key’s identity and code window or validates a challenge-response (often AES-based in newer systems).
4. If valid, the BCM energizes door-lock actuators to unlock and may flash lights or chirp the horn.
5. The alarm system disarms; interior lights may turn on. If invalid, nothing happens and events may be logged.

This process favors speed and low battery consumption while maintaining security through synchronized codes or cryptographic challenges.

What happens with passive entry (“smart” key) at the door handle?

Hands-free systems add proximity checks so the car only unlocks when the key is near the outside handle, reducing accidental or malicious unlocks. The flow below shows how this proximity verification works.

1. Touching the handle or approaching the car triggers low-frequency (≈125 kHz) antennas to wake nearby keys.
2. The car sends a challenge; the key computes a response and replies over RF/BLE. Some systems also exchange UWB signals for precise ranging.
3. The car confirms both cryptographic validity and that the key is outside and within a small distance (range-bounded, often under 1–2 m).
4. The BCM unlocks the requested door(s). Some cars double-check a second antenna zone to ensure the key is not inside.

Because the key must be physically close and authenticated, passive entry is convenient yet resistant to many simple replay attempts, especially when combined with UWB distance checks.

How the immobilizer allows the engine to start

Unlocking doors and permitting the engine to run are deliberately separate security decisions. Even with a mechanical key, a modern immobilizer will block ignition and fuel unless a valid transponder is present.

In most vehicles, a coil around the ignition cylinder or a start-button antenna powers the key’s passive RFID transponder at about 125 kHz. The vehicle and key perform a cryptographic challenge-response (protocols have included Texas Instruments DST, NXP Hitag, Megamos Crypto, and newer AES-based schemes). If the exchange succeeds, the engine control unit (ECU) enables starting. If it fails, the starter may crank but the engine will not continue running, or the starter is inhibited entirely.

Security: rolling codes, challenge-response, and modern defenses

Electronic keys must balance convenience with protection against cloning, replay, and relay attacks. The items below outline common approaches and what they defend against.

• Rolling codes: Each button press uses a new code from a synchronized sequence. Early systems (e.g., KeeLoq) were widely used; modern designs favor stronger ciphers.
• Challenge-response: The car sends a random challenge, the key returns a cryptographic response (often AES). Prevents simple replay.
• Proximity/ranging: LF field strength and antenna zoning infer location. UWB adds precise distance measurement to defeat relay attacks.
• Motion-sensing keys: Keys sleep if not moving, reducing risk of overnight relays from inside a home.
• Encrypted BLE and NFC: Phone-as-key systems pair with secure elements and can require device unlock/biometrics.
• Alarms and deadlocks: Deadlocks disable interior handles from opening a locked door; alarms trigger if forced entry occurs.

While no system is invulnerable, layered defenses—particularly UWB range bounding combined with strong cryptography—significantly raise the bar for attackers compared with older rolling-code-only designs.

What if the key fob battery is dead?

Manufacturers provide multiple fallbacks so you can still get in and drive when batteries fail. The following checklist covers the most common methods available on modern vehicles.

• Use the hidden mechanical key blade to unlock the driver’s door (often concealed in the fob).
• Hold the fob against a marked area (steering column, start button, or designated pad) so the car can read the passive transponder.
• For phone keys, tap the phone on the NFC reader area to unlock and enable start, even with a depleted battery reserve mode on some phones.
• Replace the coin-cell battery (commonly CR2032/CR2025) and re-try; consult the manual for pairing procedures if needed.

These backups rely on passive RFID or NFC that do not need the fob or phone’s main battery, ensuring you can still operate the vehicle in a pinch.

Regional frequencies and technologies

Radio rules vary by market, so vehicles ship with region-appropriate hardware. Understanding these ranges helps explain why some keys or aftermarket devices are market-specific.

• Sub‑GHz RKE: 315 MHz (North America), 433.92 MHz and 868 MHz (Europe), 902–928 MHz (US/Canada).
• Low-frequency wake-up/proximity: ~125 kHz antennas in handles and cabin.
• Bluetooth Low Energy: 2.4 GHz for phone-as-key and some passive entry functions.
• Ultra‑wideband: ~6–8.5 GHz channels for accurate distance/ranging in newer vehicles and phones.
• NFC: 13.56 MHz for tap-to-unlock/start and backup access with key cards or phones.

Manufacturers may combine several of these bands in one system to blend convenience, accuracy, and regulatory compliance across markets.

Mechanical details inside the door

Whether unlocked mechanically or electronically, the door’s latch and lock mechanisms ultimately perform the same task: retracting and engaging the latch pawl.

Turning a mechanical key rotates a cylinder cam that moves a rod or cable to the latch. Electronically, the BCM drives a small motor or solenoid actuator that flips a lock lever. “Deadlock” modes can disable the inside handle from opening the door when locked to deter theft via broken windows.

Troubleshooting common unlock problems

If your car doesn’t respond to the key, a few quick checks often resolve the issue without a service visit. The list below prioritizes simple fixes.

• Replace the fob battery and clean contacts; weak cells cause intermittent range or failure.
• Try the mechanical key and the fob’s passive start backup point to isolate RF issues.
• Move away from RF noise sources (crowded parking garages, radio towers) and try again.
• Re-sync the fob if your manual specifies a procedure after many missed presses.
• Check vehicle battery health; low voltage can disable central locking and receivers.
• If passive entry fails but buttons work, a door LF antenna or handle sensor may be faulty.

When basic steps fail, a diagnostic scan can reveal immobilizer faults, antenna failures, or BCM issues that require professional service.

Bottom line

A car key unlocks by either moving a mechanical lock or by passing cryptographic checks that command electric actuators. Modern systems layer proximity sensing, encryption, and sometimes UWB ranging to ensure the car opens only for an authenticated key that is actually nearby, while still providing mechanical and NFC backups when batteries die.

Summary

Mechanical keys physically turn a lock; remote and smart keys transmit authenticated signals that the car verifies before energizing door actuators. Passive entry uses low-frequency wake-up and RF/BLE—and increasingly UWB—to confirm both identity and distance. Separate immobilizer handshakes authorize the engine to start. Backup methods, including hidden mechanical blades and NFC tap points, keep you moving even with a dead fob or phone, while layered security mitigates replay and relay attacks.

How do key fobs open car doors?

If the fob is in range, it’s then triggered to respond to the car, sending out its own code. The car recognizes this and unlocks the doors. The vast majority of these systems are one-way with limited range. The remote sends a signal to the car, but the car does not send a signal back.

How does a car key connect to a car?

A key fob uses a low-frequency, short-range radio transmitter to send a key fob signal to the receiver inside the car. Some car key fobs can start the car engine remotely with the press of a button.

What signal does a car key give off?

radio signal
In typical designs, the car continually transmits a low-frequency (e.g., 135 kHz) radio signal to wake up any wireless keys within range.

How do car keys know which car to unlock?

Fob car keys work through their built-in Radio Frequency Identification system. The car itself has its own unique tags that the car key fob also has, so when the button on the fob is pressed, it communicates to the car that has the same tag information which in turn locks or unlocks the car.