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Home / DTC / U3021 — Control Module Wake-up Circuit B Performance

U3021 — Control Module Wake-up Circuit B Performance

Detailed page for trouble code U3021.

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U3021

Generic U — Network/User

Control Module Wake-up Circuit B Performance

Brand: Generic
Type: U — Network/User
Views: UK: 16 EN: 27 RU: 13
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Page language: EN

Causes

  • Low or dead battery / low battery voltage
  • Blown fuse or opened relay in the wake/can supply circuit
  • Open, short to ground, or short to battery on the wake wire (wake circuit B)
  • Corroded, loose or damaged connector pins at the module or BCM
  • High resistance or intermittent connection on the wake line (poor crimp or broken wire)
  • Failed module (internal wake circuit or transceiver fault)

Symptoms

  • Module(s) fail to wake or respond to scan tool
  • Intermittent loss of communication with the affected module
  • Certain vehicle functions controlled by that module are unavailable
  • No crank/no start conditions if module is required for starting
  • Random warning lights or inconsistent system behavior
  • Increased parasitic battery drain or inability for modules to go to sleep

What to check

  • Read and record freeze-frame and full DTC data with a capable scan tool; confirm U3021 is current/repeatable
  • Verify battery state-of-charge (12.6 V or higher at rest) and battery/charging health
  • Inspect and test associated fuses and relays (wake feed, module power, BCM fuses)
  • Visually inspect wiring harness and connectors for damage, corrosion, bent pins, moisture intrusion
  • Backprobe the module's wake pin(s) and measure voltage with ignition off, accessory, and crank
  • Check module ground(s) for continuity to chassis and battery negative

Signal parameters

  • Battery voltage at rest: typically ~12.4–12.8 V (fully charged). Significant deviation can prevent wake pulses.
  • Wake circuit idle voltage: near battery voltage when pulled high; near 0 V when actively pulled low (exact thresholds vehicle-dependent).
  • Wake pulse amplitude: generally near battery voltage (12 V) for wake events; pulse duration can vary by manufacturer (commonly tens to hundreds of milliseconds).
  • Wake circuit continuity: low DC resistance between module pin and fuse/BCM (typically < 5 Ω for short harness segments; large open/very high resistance indicates break).
  • High-resistance threshold: > 1 kΩ likely indicates an open/intermittent connection or corrosion on low-current wake circuits (values are vehicle-dependent).

Diagnostic algorithm

  1. Confirm code: Clear codes and reproduce conditions to verify U3021 returns. Note occurrence pattern (after sleep, on key-on, after accessory use).
  2. Battery and supply: Measure battery voltage at rest and during crank. Charge or replace battery if below specification; verify charging system.
  3. Fuses and relays: Check and test all fuses/relays feeding the module wake and power circuits. Replace if faulty.
  4. Visual inspection: Inspect wiring, harness routing and connectors to the module and BCM for damage, corrosion or water intrusion.
  5. Backprobe and measure: With connector connected, backprobe the wake circuit B pin and monitor voltage while cycling key and door inputs to generate wake events. Compare to expected behavior.
  6. Scope check: If available, use an oscilloscope to observe wake pulse shape, amplitude and timing. Look for noise, no pulses, or weak pulses.
  7. Ground check: Verify module ground(s) have low resistance (
  8. Isolate parasitic loads: If battery drains or unexpected wake events occur, perform parasitic current test to identify aftermarket or internal loads.
  9. Connector and pin repair: Repair or replace damaged connectors, terminals, or wiring. Ensure proper crimps/seating and dielectric grease for corrosion-prone areas.
  10. Module swap/software: If wiring and power are good and wake signal absent, consider reprogramming or replacing the module or its transceiver per manufacturer guidance. If available, test with a known-good module.
  11. Verify repair: Clear codes and road/test or cycle vehicle through sleep/wake conditions to confirm the code does not return.

Likely causes

  • Battery voltage below specification preventing wake pulses
  • Open or corroded connector at the module or ground
  • High-resistance wake wire (broken strand or poor crimp)
  • Module internal failure (transceiver or wake circuitry)
  • Blown fuse or failed relay in the module supply/wake feed

Fault status

⚠️ Status
Control Module Wake-up Circuit B Performance — module failed to meet expected wake-up signal performance on wake circuit B; communication/wake event missing or out of specification.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5 - 3.0 hours

Similar codes

U0100 U0121 U0131 U0140 U0283 U0300
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