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DTC codes / U30xx · Network wiring and bus faults / U301F

U301FControl Module Wake-up Circuit B Low

U301F indicates that a control module on the vehicle's network has detected a low-voltage condition on a wake-up circuit, which is the signal line used to rouse another module from sleep mode. The computer sets this code when the wake-up circuit B voltage stays below the expected threshold for longer than the module's programmed delay, preventing reliable communication or activation of the affected module.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaNetwork wiring and bus faults

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the affected module is non-critical.
  • Intermittent failure of one or more electronic features, such as power windows, radio, or climate control.
  • A module that stays awake and drains the battery, leading to a no-start condition after sitting.
  • Multiple unrelated warning lights or communication codes appearing at the same time.
  • Delayed or inconsistent response when pressing a button that should wake a module, like a door handle or ignition switch.

Common causes, most likely first

  1. A chafed or broken wire in the wake-up circuit B harness, often at a door hinge or under a seat.
  2. Corroded or loose terminals at the control module connector or a splice point in the wake-up circuit.
  3. A faulty control module that is internally shorted to ground on its wake-up input, pulling the circuit low.
  4. Water intrusion into a connector or module housing, causing a low-resistance path to ground.
  5. A failed ignition switch or push-button start module that fails to supply the correct wake-up voltage.
  6. A short to ground in a related circuit that shares the same wake-up line, such as a body control module output.

How to diagnose and fix it

  1. Verify the code with a scan tool and record all stored and pending codes to understand the full network context.
  2. Inspect the wake-up circuit B wiring for visible damage, corrosion, or loose connectors at each module along the path.
  3. Measure voltage on the wake-up circuit B at the affected module with the key on and off, comparing to the service specification.
  4. Perform a voltage drop test on the ground and power feeds to the module to rule out a shared ground fault.
  5. Disconnect the suspected module and check whether the low-voltage condition clears, isolating an internal short.
  6. Repair or replace the faulty wire, connector, or module as indicated by the tests, then clear codes and verify the circuit wakes correctly.
  7. Road-test the vehicle and monitor live data to confirm the wake-up signal toggles as designed and the code does not return.

Wake-up circuit faults that fool technicians

  • A wake-up circuit that stays low can be caused by a module that never goes to sleep, which drains the battery and sets the code only after the vehicle sits for hours.
  • Technicians often replace the module named in the code when the real fault is a chafed wire in a door boot or a corroded splice pack under the carpet.
  • This code can be set by a completely different module on the same network if it shares the wake-up line, so always check the entire circuit rather than just the module that reported the fault.
  • A weak battery or poor ground can mimic a wake-up circuit fault because the module may not see the expected voltage rise when the wake-up signal is applied.
  • After repairing the circuit, some modules require a sleep-cycle relearn or a battery disconnect for several minutes before the code will clear permanently.

Can I keep driving with U301F?

You can usually drive the vehicle, but avoid leaving it parked for long periods until the fault is repaired because a module that stays awake can drain the battery and leave you stranded. If the warning light is accompanied by a dead battery or multiple electrical failures, have the circuit inspected promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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