U0040
Communication Bus Off
A control module has counted enough transmit errors to enter the bus-off state defined by ISO 11898 and stops transmitting on the bus. Multiple modules typically lose communication and several warning lamps illuminate at once.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- CAN line shorted to supply or groundCommon
- Terminating resistor failedPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- CAN Bus Wiring
- CAN Terminating Resistor
U0040 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.