What it means
Wake-up signal B timing or pattern does not match expected behavior. 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 the timing or pattern on this path and sets U3021 when it does not match the expected behavior. The circuit may have voltage, yet a noisy or mistimed signal can still prevent reliable wake-up.
Common causes
Most likely first.
- Wake signal noise or glitch
- 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, noting whether the fault coincides with ignition-state changes.
- Monitor wake-up circuit B status or waveform during key transitions and compare its timing and pattern with the manufacturer's specification.
- Inspect the second wake-up path for loose terminals, corrosion, poor splices, or harness routing that could introduce interference.
- Check circuit B continuity and connection quality, then monitor for glitches while gently moving the harness if the fault is intermittent.
- If circuit B is sound, verify gateway routing and CAN network termination against the manufacturer's specification.
Common fixes
- Secure loose circuit B connectors and repair corroded terminals.
- Repair damaged wiring or splices causing intermittent noise on the second wake-up path.
- Correct gateway routing or configuration if it disrupts circuit B timing.
- Repair a confirmed CAN network or termination fault if the circuit signal is otherwise correct.
Can I keep driving?
Intermittent loss of the module can disable its functions or trigger limp mode in dependent systems. Avoid driving if the affected function is safety-related or the vehicle behaves unpredictably.
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.