What it means
Wake-up signal B voltage below expected range. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
Wake-up circuit B is the second signal path that tells a control module to become active. The module monitors its voltage and sets U301F when circuit B falls below the expected range, consistent with a short to ground. This can prevent the module from waking or communicating through that path.
Common causes
Most likely first.
- Wiring shorted to ground
- Gateway module routing CAN messages incorrectly
- CAN terminating resistor (120 Ω) drifted out of spec
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
- Save freeze-frame data and scan all modules for communication faults to identify the affected module and when circuit B went low.
- Check live data for wake-up circuit B status or voltage, if available, against the ignition state.
- Inspect circuit B wiring, connectors, and grounds for chafing, moisture, pin damage, or contact with grounded metal.
- With the circuit isolated as directed by the manufacturer, test for a short from wake-up circuit B to ground.
- If the wiring passes, check gateway routing and CAN network condition, including termination resistance, against the manufacturer's specification.
Common fixes
- Repair wiring that shorts wake-up circuit B to ground.
- Clean or repair damaged circuit B connectors and terminals.
- Repair a poor ground or splice affecting the second wake-up path.
- Correct gateway routing or repair a confirmed CAN termination fault if circuit B itself tests good.
Can I keep driving?
The affected module may fail to wake and its functions may be unavailable, with dependent systems potentially entering limp mode. Avoid driving if the missing function affects safe vehicle control.
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.