CAN Protocol

The CAN protocol (Controller Area Network) is the set of rules that the electronic control units of a vehicle follow to exchange messages over a shared network. It defines how each message frame is built, how priority is decided when two units transmit at once, and how errors are detected, so that engine, braking and dashboard data stay consistent.

CAN protocol versus CAN bus

The two terms are often used interchangeably, but they describe different layers. The CAN bus is the physical network: a twisted pair of wires linking the control units. The CAN protocol is the language spoken on those wires. One way to picture it: the bus is the road, the protocol is the highway code that decides who goes first and what every sign means.

How a CAN frame works

Every message on the network is a short frame broadcast to all units. Each unit decides whether the frame concerns it. A frame contains:

• An identifier: it names the content (engine speed, wheel speed, fuel level) rather than the sender, and it sets the priority.
• Arbitration: when two units start transmitting together, the frame with the lower identifier wins without being corrupted, so a brake signal is never delayed by a comfort function.
• A data field: a small payload carrying the actual values. The newer CAN FD variant allows larger payloads and faster data rates.
• Error checks: a checksum and an acknowledgement from the receivers, so faulty frames are detected and sent again.

Higher-layer standards: OBD and J1939

The CAN protocol only says how bytes travel. What those bytes mean is defined by higher-layer standards. On cars and light vans, on-board diagnostics (OBD) standardises requests for emissions-related data and fault codes. On heavy trucks, buses and many construction machines, SAE J1939 defines standard parameters, such as engine speed, total fuel used, engine hours or distance, with a common naming scheme across manufacturers. Beyond these shared signals, most messages remain manufacturer-specific and need decoding.

Why it matters for a company fleet

• Data quality: values read from the vehicle network, such as odometer or fuel level, are more reliable than estimates calculated from GPS alone.
• Mixed fleets: light vehicles and heavy vehicles do not speak the same higher-layer standard, so a telematics setup must handle both.
• Safe access: reading the network should be passive and done with approved equipment, to avoid any interference with vehicle systems or warranty issues.

How Dadycar helps

Dadycar is a French fleet management software that turns vehicle network data into usable fleet information without asking you to decode frames. On compatible connected vehicles, its telematics works without a box: the manufacturer's embedded telematics reads the vehicle network and shares the data. An optional tracker covers other vehicles. Mileage, alerts and fuel consumption tracking then appear in one platform, hosted in France with OVHcloud. The Fleet plus Telematics plan costs 16 € excl. VAT per vehicle per month.

FAQ

Is the CAN protocol the same on every vehicle?
The base protocol is standardised, but the meaning of most messages varies by manufacturer and model. Only signals covered by standards such as OBD or J1939 are shared.

What is J1939 used for?
J1939 is a higher-layer standard built on CAN for heavy vehicles. It gives common names and formats to parameters such as engine speed, fuel used and engine hours.

Can a fleet read CAN data without installing hardware?
Yes, on compatible connected vehicles: the manufacturer's built-in module reads the network and shares selected data with authorised platforms.