U0074
Control Module Communication Bus "B" Off
A control module has shut its bus B interface off because the bus is non-functional. Bus B is typically a secondary CAN segment (often body or chassis) and a failure disables all modules connected to it.
- 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 B line shorted to supply or groundCommon
- One failing module pulling the bus downPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- CAN Bus Wiring
- CAN Terminating Resistor
- Control Module
U0074 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.