U0091
CAN Communication Bus "C" Off
Bus C (third CAN bus, often used for chassis or infotainment) has entered the bus-off state. All modules on Bus C lose communication.
- 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.
- Bus C shorted to supply or groundCommon
- Bus C terminating resistor failedPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- CAN Bus Wiring
- CAN Terminating Resistor
U0091 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.