What it means
Wake-up signal A voltage below expected range — short to ground. 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 A carries the signal that tells a control module to become active. The receiving module monitors this line and sets U301B when its voltage falls below the expected range, consistent with a short to ground. If the module does not wake or communicate, functions that depend on its data may be unavailable.
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 which module is not responding and when the fault occurred.
- Compare wake-up circuit A status or voltage in live data, if available, with the ignition state and the module’s expected response.
- Inspect the circuit A wire, connectors, and grounds for chafing, pin damage, moisture, or contact with grounded metal.
- With the circuit isolated as directed by the manufacturer, check for a short from wake-up circuit A to ground.
- If the wiring passes, check gateway routing and CAN network condition, including termination resistance, against the manufacturer's specification.
Common fixes
- Repair chafed or grounded wiring on wake-up circuit A.
- Clean, secure, or repair damaged connectors and ground connections.
- Correct gateway configuration or routing if diagnostic evidence identifies it as the source.
- Repair the CAN network or replace an out-of-spec terminating resistor after confirming the fault.
Can I keep driving?
Avoid relying on functions that depend on the affected module; related warnings, shutdowns, or limp mode may occur. If steering, braking, or another safety-related function is affected, stop driving and arrange inspection.
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.