What happens if a bus-off error occurs in a CAN controller while a car is in motion?

Question

I know that in a CAN controller if the error count reaches some threshold (say 255), bus off will occur which means that a particular CAN node will get switched off from the CAN network. So there won't be any communication at all. But what if the above said scenario happens while the car is moving which contains the ECU (includes the CAN controller)?

Is there any auto-recovery mechanism in a CAN controller to avoid any of the above situations?

Answer 1

During bus off, the node will be isolated.

CAN waits for the mandatory time period, 128 x 11 bits (1408 bits - 5.6 ms for a 250 kbit/s system) of time, and then tries to re-initialize the node.

Answer 2

Yes, if a CAN Tx error count reaches 255, a node will turn off and potentially reset itself. A good implementation will not continue resetting a node if the problem persists.

In addition to this safety mechanism, ECU's (electric control units) also time the duration between valid transmissions of the messages they expect to receive. Therefore, if the engine controller goes offline, nearly every ECU in the vehicle will report "Lost Communication with the Engine Controller."

Typically, these type of CAN problems are identified by DTC's (diagnostic trouble codes) beginning with U, like this one: http://www.obd-codes.com/u0115

Depending on the severity of the issue, the vehicle might enter a "limp home" mode, or might be totally disabled. Problems with the CAN bus on a vehicle are extremely rare, unless there has been some tampering.

Answer 3

Nothing spectacular will happen, even if the Engine Control Unit looses CAN communication. The car will continue running.

Answer 4

When bus-off occurs, the CAN network isolates that node and then resets that node which can able to start communication.