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DTC codes / U00xx · Network wiring and bus faults / U0026

U0026Low Speed CAN Communication Bus (-) High

U0026 indicates that the vehicle's low-speed Controller Area Network (CAN) bus, specifically the CAN (-) wire, has been detected at an abnormally high voltage level, which disrupts communication between control modules. The computer monitors the differential voltage between the CAN (+) and CAN (-) lines and sets this code when the negative line's signal is outside the expected range, often due to a short to voltage or a faulty transceiver.

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

Symptoms you may notice

  • Multiple warning lights on the dashboard, such as check engine, ABS, and airbag, due to lost communication between modules.
  • Intermittent or complete failure of non-critical systems like radio, climate control, or power windows.
  • The engine may start and run normally, but other electronic features may behave erratically.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • Diagnostic trouble codes (DTCs) in multiple modules indicating lost communication with other controllers.
  • The vehicle may enter a limp mode or fail to start if the fault affects critical modules.

Common causes, most likely first

  1. A short to voltage on the CAN (-) wire, often caused by chafed insulation contacting a power wire.
  2. A faulty control module internally shorting the CAN (-) line to voltage.
  3. Corroded or damaged connectors at a module, allowing voltage to bleed onto the CAN (-) circuit.
  4. Aftermarket accessories improperly wired into the CAN bus, introducing voltage on the CAN (-) line.
  5. A defective CAN transceiver within one of the modules on the low-speed bus.
  6. Water intrusion into a module or harness, causing conductive paths to voltage sources.
  7. A poor ground connection for a module, leading to abnormal voltage levels on the CAN (-) line.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for related communication codes in all modules to identify which bus is affected.
  2. Inspect the CAN (-) wire for visible damage, chafing, or corrosion, especially near connectors and known harness routing points.
  3. Disconnect modules one by one to see if the high voltage on CAN (-) disappears, isolating the faulty module.
  4. Measure resistance and voltage on the CAN (-) circuit with a digital multimeter (DMM) to confirm a short to voltage.
  5. Check for aftermarket accessories wired to the CAN bus and remove or repair any improper connections.
  6. Repair or replace damaged wiring, connectors, or the faulty module as needed, then clear codes and verify communication.
  7. Perform a road test and monitor live data to ensure the code does not return.

When CAN (-) Reads High, Check These First

  • A common oversight is assuming the code points to the CAN (-) wire itself, but often the fault lies in a module that is internally shorting the line to voltage.
  • Aftermarket alarms, remote starters, or telematics devices tapped into the CAN bus are frequent culprits for voltage intrusions on the CAN (-) line.
  • The low-speed CAN bus often continues to operate with degraded performance, so the only symptom may be a single warning light and stored codes in multiple modules.
  • A faulty ground for a module can cause the CAN (-) reference voltage to rise, mimicking a short to voltage, so always check ground integrity before replacing parts.
  • This code can be set by a module that remains powered up after the ignition is off, backfeeding voltage onto the CAN (-) line and disrupting sleep mode.

Can I keep driving with U0026?

If the vehicle runs and drives normally, you can drive it to a repair shop, but avoid using non-essential electronic features and be prepared for possible intermittent failures. If multiple warning lights are on or the engine runs poorly, stop driving and have the vehicle towed to prevent safety risks.

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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