U3021 indicates a malfunction in the Wake‑Up Circuit B that the control module uses to detect when the vehicle’s electrical system is powered. The circuit is not providing the expected signal, preventing proper communication between modules.
Corroded connectors, broken wires, or pinched harnesses can interrupt the signal.
A defective sensor that provides the wake‑up voltage to the module will trigger the code.
A bad ground strap or insufficient battery voltage can prevent the circuit from achieving the required level.
Rarely, the module’s internal driver circuitry can fail, but this is less common than wiring problems.
Only after ruling out hardware issues might a software update be considered, though it is seldom the root cause.
No. In the vast majority of cases the problem is a wiring or sensor fault. Replacing the PCM is rarely necessary and should be a last resort after confirming the hardware is functional.
Yes. Insufficient battery voltage can prevent the wake‑up circuit from achieving the required level, so checking battery health is a useful early step.
Typically, the code will reappear after a drive cycle if the underlying fault remains, because the PCM continuously monitors the wake‑up circuit.
No. U3021 is a communication‑level diagnostic code, not an emissions or catalyst fault.