OBD-II code · Network (extended)
U3021 Control Module Wake-up Circuit B Performance
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.
Is it safe to drive with U3021?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyWake signal noise or glitch
- Less likelyGateway module routing CAN messages incorrectly
- Less likelyCAN 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
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Often, with basic tools
- Typical labour
- 0.5–3 h
- Emissions test
- Usually unaffected
- Parts involved
- Wakeup Circuit B
Related codes
- U301DControl Module Wake-up Circuit A Performance
- U301EControl Module Wake-up Circuit B/Open
- U301FControl Module Wake-up Circuit B Low
- U3020Control Module Wake-up Circuit B High
- U3022Ignition Input Accessory/Off
- U3023Control Module Input Power Circuit A - Ignition Input On/Start Circuit Correlation
- U3024Control Module Input Power Circuit B - Ignition Input On/Start Circuit Correlation
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.