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DTC codes / U30xx · Network wiring and bus faults / U3020

U3020Control Module Wake-up Circuit B High

U3020 indicates that a control module responsible for waking other modules on the vehicle's communication network has detected an abnormally high voltage on its wake-up circuit B. The module monitors the wake-up line and sets this code when the signal remains above the expected threshold, which can prevent proper sleep and wake cycles across the network.

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 fault is intermittent.
  • Battery drain when the vehicle sits because modules fail to enter sleep mode.
  • Intermittent no-start or delayed start due to modules not waking correctly.
  • Multiple unrelated warning lights or communication codes appearing together.
  • Accessories or features controlled by networked modules may act erratically or stay active after shutdown.

Common causes, most likely first

  1. A short to voltage on the wake-up circuit B wiring, often from chafed insulation or pinched harness.
  2. A faulty control module internally shorting the wake-up line high.
  3. Water intrusion or corrosion in a connector on the wake-up circuit causing unintended voltage.
  4. Aftermarket accessories improperly tapped into the wake-up circuit or nearby power feeds.
  5. A failed or improperly installed relay that backfeeds voltage onto the wake-up line.
  6. Loose or damaged ground connections causing reference voltage shifts on the circuit.

How to diagnose and fix it

  1. Verify the code with a scan tool and record all stored and pending codes to establish the full network fault picture.
  2. Inspect the wake-up circuit B wiring and connectors for damage, corrosion, or aftermarket taps, repairing as needed.
  3. Measure voltage on the wake-up circuit B at the module connector with the key off and on, comparing to manufacturer specifications.
  4. Disconnect suspect modules one at a time to see if the high voltage on the wake-up line disappears, isolating the source.
  5. Check relays and power distribution points associated with the wake-up circuit for backfeeding or stuck contacts.
  6. Repair or replace the faulty module or wiring, then clear codes and perform a sleep-cycle test to confirm the network powers down correctly.
  7. Road test and monitor for recurrence, verifying that all modules enter sleep mode and the battery drain issue is resolved.

Wake-up circuit faults that fool technicians

  • A common misdiagnosis is replacing the module that sets the code when the actual fault is a short to voltage on the wake-up line caused by another module or aftermarket device.
  • The code can be set by a module that is simply reacting to a high voltage present on the shared wake-up bus, so the module reporting the fault is not always the defective one.
  • Intermittent battery drain complaints often trace back to this code, but the drain may only occur after the vehicle has been parked for several hours, making it hard to reproduce in the shop.
  • Water intrusion in a connector far from the module can introduce voltage on the wake-up circuit through corrosion-induced leakage, so a thorough visual inspection of all related connectors is essential.
  • Some vehicles use a wake-up signal that is pulsed rather than steady-state, and a meter may show a misleading average voltage; an oscilloscope is needed to see the true signal integrity.

Can I keep driving with U3020?

The vehicle is generally safe to drive short distances, but avoid leaving it parked for extended periods until the fault is repaired because the battery may drain and leave you stranded. If multiple warning lights or starting issues appear, have the vehicle 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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