What it means
A control module has detected that communication bus B is off. The code does not specify which module or the underlying cause.
At a glance
- Usually lights the warning light of the system that lost communication
- The engine or gearbox may go into limp mode
How it works
Communication bus B carries data among the modules connected to that network. U0074 means a module has detected that Bus B is off, and further diagnosis must determine whether a Bus B wire, connected module, termination, or gateway path caused the outage.
Common causes
Most likely first.
- Bus B line shorted to supply or ground
- One failing module pulling the bus down
- Gateway module routing CAN messages incorrectly
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
How to diagnose it
- Scan all reachable modules and save freeze-frame and network codes to identify which devices on Bus B are not responding.
- Inspect Bus B wiring and connectors for damage and check for shorts to power or ground, paying particular attention to the Bus B route shown in the wiring diagram.
- With power isolated as specified by the manufacturer, test Bus B conductors for continuity and shorts.
- Measure Bus B termination resistance at the specified points and compare it with the manufacturer's specification.
- If the conductors and termination are sound, isolate Bus B modules one at a time and check gateway routing using the manufacturer's procedure.
Common fixes
- Repair Bus B wiring shorted to power or ground.
- Clean, secure, or replace a damaged Bus B connector or splice.
- Repair or replace a failed Bus B terminating component identified by measurement.
- Repair or replace the Bus B module or gateway found to be taking the network offline.
Can I keep driving?
Avoid driving if the outage affects essential controls, warning lights indicate a safety function is unavailable, or the vehicle enters limp mode. Bus B loss can disable the specific functions that rely on its connected modules.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.