U0173
Communication Bus "C" Off
A control module has detected that bus C is non-functional and shut its bus C interface off. Bus C is typically a tertiary segment used for diagnostics or specialized subsystems; modules on the segment fall silent simultaneously.
- 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
- One failing module pulling bus C downPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- CAN Bus Wiring
- Control Module
U0173 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.