In automotive context, CAN stands for Controller Area Network. It’s a communication system that lets the dozens of computers inside your vehicle talk to each other over a shared pair of wires instead of requiring a separate wire for every single connection. Bosch developed CAN in 1986 to replace the increasingly unmanageable point-to-point wiring harnesses that cars relied on at the time, and it has since become the standard communication backbone in virtually every modern vehicle.
The Problem CAN Was Built to Solve
A modern car is packed with small computers called electronic control units, or ECUs. Your engine has one, your transmission has one, your anti-lock brakes have one, and so on. A premium vehicle today can contain over 100 ECUs, and a level-4 autonomous vehicle may have more than 150. Advanced driver assistance systems alone can require up to 70 interconnected ECUs.
Before CAN, each of these modules needed its own dedicated wires to communicate with every other module it interacted with. As cars added more electronics through the 1970s and 1980s, the wiring harness grew heavier, more expensive, and harder to troubleshoot. CAN replaced that tangle with a single shared network: two wires that every module connects to, taking turns sending and receiving messages.
How the Two-Wire System Works
A CAN bus uses just two signal wires, called CAN High and CAN Low. These wires carry opposite voltage changes at the same time. When a module sends a signal, CAN High is driven toward 3.5 volts while CAN Low is driven toward 1.5 volts, creating a 2-volt difference between them. When nobody is transmitting, both wires rest at the same voltage, producing a 0-volt difference.
This “differential signaling” approach is what makes CAN so reliable in the electrically noisy environment inside a car, surrounded by spark plugs, electric motors, and radio waves. Any interference that hits the wiring affects both wires equally. Since the receiver only cares about the difference between the two wires, the noise cancels itself out. The wires are twisted together in a shielded cable to further reduce interference, following the same principle used in ethernet cables.
At each end of the bus, a 120-ohm resistor connects the two wires together. These termination resistors prevent electrical reflections from bouncing back along the wire and corrupting the signal. Without them, the system loses reliability. At low speeds and short distances, you might get some data through with dropped frames, but at higher speeds the reflections will overwhelm the actual messages.
How Modules Take Turns Communicating
Every message on the CAN bus is broadcast to all modules at once. There’s no central controller deciding who speaks when. Instead, each message carries a priority number called an arbitration ID. When two modules try to transmit at exactly the same time, the bus automatically resolves the conflict without destroying either message. The module with the lower arbitration ID (higher priority) continues transmitting, while the other backs off and tries again. This happens at the bit level, so the winning message isn’t delayed at all.
This priority system is why CAN is so well suited to vehicles. Safety-critical messages, like those from the anti-lock brake system, are assigned low arbitration IDs so they always get through first. Less urgent messages, like an interior light status update, wait their turn.
Each standard CAN message can carry up to 8 bytes of data, along with a 15-bit checksum that lets the receiver verify the message wasn’t corrupted during transmission. Every module that successfully receives a message sends back an acknowledgment bit, so the sender knows the message got through.
Classic CAN vs. CAN FD
The original CAN standard, now called Classic CAN, tops out at 1 megabit per second and carries a maximum of 8 bytes per message. That was plenty for the 1990s and 2000s, but as vehicles added cameras, radar, and more sophisticated control systems, the bus needed more bandwidth.
CAN FD (Flexible Data-rate) is the updated version. It increases the maximum payload from 8 bytes to 64 bytes per message, and it can switch to a faster bit rate during the data portion of each message, up to eight times the speed used during arbitration. This means CAN FD moves significantly more data per message while remaining backward-compatible with the same basic wiring and network structure.
Where You’ll Find It in Your Car
If your vehicle has an OBD-II diagnostic port (the standard connector where mechanics plug in scan tools), CAN is accessible on pin 6 (CAN High) and pin 14 (CAN Low). Since 2008, all cars sold in the United States are required to use CAN as the communication protocol for OBD-II diagnostics. That’s why aftermarket scan tools, performance tuners, and diagnostic apps can read data from your vehicle: they’re communicating over the CAN bus.
Inside the vehicle, there are typically multiple CAN buses running at different speeds. A high-speed bus handles powertrain and safety systems, while a slower bus connects comfort features like climate control and seat adjustments. A gateway module bridges the two so they can share information when needed.
Security Limitations
CAN was designed in the 1980s with reliability in mind, not security. The protocol has no encryption and no way to verify which module actually sent a message. Any device connected to the bus can read every message being transmitted and can inject its own messages that other modules will accept without question.
This wasn’t a concern when the bus was physically sealed inside a vehicle. But as cars added Bluetooth, Wi-Fi, cellular connections, and USB ports, researchers demonstrated that attackers could potentially reach the CAN bus through these external interfaces. The broadcasting nature of the protocol means a single compromised connection point gives access to messages across the entire network. Adding encryption is difficult because the CAN bus has very limited bandwidth and the communication needs to happen in real time with no delays. Automakers have responded with gateway modules that filter traffic between internal and external-facing networks, and with intrusion detection systems that monitor for abnormal message patterns.