What CAN actually coordinates
Controller Area Network is a broadcast bus. ECUs do not send a frame to one named destination; they place an identifier and payload on the shared network, and every connected node decides whether the frame matters to it.
The identifier represents message priority and meaning by project convention. In an 11-bit network, identifier 0x100 wins arbitration over 0x300 because dominant bits overwrite recessive bits without corrupting the winning frame.
Frames, timing, and bus load
A classical CAN data frame carries up to eight payload bytes. CAN FD extends the payload and can use a faster data phase when the network and interfaces support it. The nominal bit rate still controls arbitration.
Bus load is not only a percentage. Engineers should also inspect message periods, bursts, high-priority traffic, error frames, retransmissions, and whether critical messages still meet their timing budget under worst-case conditions.
A disciplined first diagnostic pass
Confirm the expected bit rate, physical termination, adapter channel, and whether the bus is active. Then record a trace without aggressive filtering so error frames, unexpected identifiers, and timing gaps remain visible.
Group messages by identifier, measure period and jitter, decode the known DBC, and compare the evidence against the ECU state. A payload that looks wrong can be a database issue, endianness mismatch, stale signal, or genuine software fault.