What it means
Open circuit on wake-up signal line B (second wake-up path). 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 wake-up path used to signal a control module to become active. The module monitors this path and sets U301E when circuit B is open, so the expected signal cannot reach it. The module may fail to wake or communicate even if circuit A is intact.
Common causes
Most likely first.
- Wake-up wire broken
- 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 module affected by the missing circuit B signal.
- Check live data for circuit B status, if available, while switching the ignition through the states that should wake the module.
- Inspect the circuit B wire and connectors for breaks, pulled pins, corrosion, or damage at flex points and splices.
- With the circuit de-energized as directed by the manufacturer, check continuity through wake-up circuit B and isolate any open section.
- If circuit B wiring is continuous, check gateway routing and CAN network termination against the manufacturer's specification.
Common fixes
- Repair or replace the broken section of wake-up circuit B wiring.
- Reconnect or repair loose, corroded, or damaged circuit B terminals.
- Repair a failed splice or connection in the second wake-up path.
- Correct gateway routing or address a confirmed CAN termination fault if circuit B tests good.
Can I keep driving?
The module may not wake, leaving its dependent functions unavailable; do not rely on those functions until the fault is resolved. Stop driving if a safety-related system is affected.
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.