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DTC codes / U30xx · Network wiring and bus faults / U301B

U301BControl Module Wake-up Circuit A Low

U301B indicates that a control module responsible for waking up other modules on the vehicle's communication network has detected a low voltage condition on the wake-up circuit it monitors. The module sets this code when the wake-up line, which is normally held at a defined voltage when inactive, drops below the expected threshold, suggesting a short to ground, an open circuit, or a failed module pulling the line low.

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

Symptoms you may notice

  • The warning light (malfunction indicator lamp) illuminates, often with no other noticeable symptoms.
  • One or more electronic modules may fail to power up or communicate, leading to inoperative accessories such as radio, climate control, or power windows.
  • Intermittent no-start condition if the wake-up circuit is part of the immobilizer or powertrain enable logic.
  • Battery drain if the low wake-up circuit keeps modules awake when they should be asleep.
  • Unexplained electrical glitches, such as flickering interior lights or random dashboard warnings.
  • Often no symptoms besides the warning light, especially if the fault is intermittent or affects a non-critical module.

Common causes, most likely first

  1. A short to ground in the wake-up circuit wiring, often due to chafed insulation or pinched harness.
  2. An open circuit or high resistance in the wake-up line, such as a broken wire or corroded connector.
  3. A failed control module that is internally shorted and pulling the wake-up line low.
  4. Water intrusion or corrosion in a connector on the wake-up circuit, especially in areas prone to moisture.
  5. A faulty ground connection for the module that monitors the wake-up circuit, causing a false low reading.
  6. Aftermarket accessories improperly tapped into the wake-up circuit, altering its voltage.
  7. A defective relay or fuse that supplies power to the wake-up circuit, though less common.

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes and freeze-frame data to understand the fault context.
  2. Obtain the vehicle-specific wiring diagram for the wake-up circuit and identify all modules and connectors on that line.
  3. Visually inspect the wake-up circuit wiring and connectors for damage, corrosion, or loose pins, especially near moving components or moisture-prone areas.
  4. Disconnect each module on the wake-up circuit one at a time and measure the voltage on the line to see if the low condition disappears, isolating a faulty module.
  5. Measure resistance to ground on the wake-up wire with all modules disconnected; a very low resistance indicates a short to ground that must be traced and repaired.
  6. Check the power and ground supplies to the monitoring module to rule out a false low reading caused by a bad ground.
  7. Repair or replace the faulty wiring, connector, or module as identified, then clear the code and verify the wake-up circuit operates correctly with a multimeter or oscilloscope.

Wake-up circuit faults often misdiagnosed

  • A wake-up circuit is not the same as a communication bus; it is a discrete signal line that tells modules to power up, so a fault here can mimic a dead module or a CAN bus failure.
  • Many technicians replace the monitoring module first, but the code often originates from a shorted module elsewhere on the same wake-up line pulling the voltage low.
  • Intermittent low voltage on the wake-up circuit can be caused by a module that fails only when cold or wet, so testing when the fault is active is critical.
  • Aftermarket alarms or remote starters frequently tap into wake-up circuits and can introduce a low-voltage condition that sets this code.
  • The code does not always mean the wake-up circuit itself is defective; a weak battery or charging system issue can cause modules to see low voltage during cranking and set a false U301B.

Can I keep driving with U301B?

If the vehicle starts and runs normally, you can drive it to a repair shop, but avoid long trips or relying on accessories that may be affected. If the engine cranks but does not start, or if multiple electrical systems are dead, do not drive and have the vehicle towed.

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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