U0009
High Speed CAN Communication Bus (-) Shorted to Bus (+)
CAN-High and CAN-Low are shorted together. The differential signal collapses to zero and the bus controller can no longer separate dominant from recessive bits, so all communication stops.
- 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-H and CAN-L shorted togetherCommon
- Pinched harness behind a modulePossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- CAN High Wire
- CAN Low Wire
- Module Connector
U0009 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.