U3021

Code

Generic U — Network/User

Control Module Wake-up Circuit B Performance

Brand: Generic

Type: U — Network/User

Views: UK: 24 EN: 45 RU: 22

AI status

Completed

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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

Confirm code: Clear codes and reproduce conditions to verify U3021 returns. Note occurrence pattern (after sleep, on key-on, after accessory use).
Battery and supply: Measure battery voltage at rest and during crank. Charge or replace battery if below specification; verify charging system.
Fuses and relays: Check and test all fuses/relays feeding the module wake and power circuits. Replace if faulty.
Visual inspection: Inspect wiring, harness routing and connectors to the module and BCM for damage, corrosion or water intrusion.
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.
Scope check: If available, use an oscilloscope to observe wake pulse shape, amplitude and timing. Look for noise, no pulses, or weak pulses.
Ground check: Verify module ground(s) have low resistance (
Isolate parasitic loads: If battery drains or unexpected wake events occur, perform parasitic current test to identify aftermarket or internal loads.
Connector and pin repair: Repair or replace damaged connectors, terminals, or wiring. Ensure proper crimps/seating and dielectric grease for corrosion-prone areas.
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.
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