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DTC codes / U00xx · Network wiring and bus faults / U0008

U0008High Speed CAN Communication Bus (-) High

U0008 indicates that the high-speed Controller Area Network (CAN) data bus, specifically the CAN negative (CAN-) circuit, has been detected by a control module as being at an abnormally high voltage level. The module monitoring this bus expects the CAN- line to rest near 2.5 volts and pull low during communication, so a sustained high reading points to a short to voltage or an open circuit on that wire.

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

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) or a dedicated communication bus warning light
  • Multiple unrelated warning lights on the dashboard because several modules share the same bus
  • Intermittent or complete failure of one or more electronic systems such as stability control, transmission shifting, or infotainment
  • The engine may start and run normally with no drivability complaint, especially if the fault is intermittent
  • No symptoms besides the warning light when the fault is stored but not currently active
  • A no-start condition if the affected module is the engine control module and the bus fault prevents communication

Common causes, most likely first

  1. A short to battery voltage or a short to a 5-volt reference on the CAN- wire, often from chafed insulation against a power feed
  2. An open circuit in the CAN- wire, such as a broken conductor at a connector or a corroded terminal
  3. A failed control module internally shorting the CAN- line high
  4. Water intrusion into a connector or module causing corrosion and altered resistance on the bus
  5. Aftermarket accessories improperly spliced into the CAN wiring, pulling the CAN- line to an unexpected voltage
  6. A faulty terminating resistor inside a module that disturbs the bus voltage levels
  7. Damaged or pinched wiring harness from collision repair or rodent damage

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes in every module to identify which modules are online and which are not
  2. Measure the CAN- voltage at the data link connector (DLC) with the key on and engine off, comparing it to the expected 2.5-volt rest level
  3. Disconnect modules one at a time to see if the high CAN- voltage returns to normal, isolating the faulty module or wiring section
  4. Inspect the CAN- wire for continuity and shorts to voltage using a digital multimeter and a wiring diagram for the specific vehicle
  5. Check for aftermarket wiring splices, corrosion at connectors, and signs of water intrusion along the harness
  6. Repair or replace the damaged section of harness or the faulty module, then clear codes and verify communication on all modules
  7. Perform a road test with the scan tool recording live data to confirm the fault does not return under vibration or temperature changes

What the CAN- voltage reading really tells you

  • A CAN- voltage that reads near 5 volts often indicates a short to a 5-volt reference circuit rather than a short to battery voltage, which would read closer to 12 volts.
  • Many technicians replace the module that sets U0008, but the code frequently originates from a wiring fault elsewhere on the bus that only shows up when the harness is flexed.
  • An open CAN- circuit can mimic a short to voltage on a scan tool because the bias voltage from other modules pulls the floating line high.
  • Aftermarket alarms, remote starters, and dash cameras are common culprits when they are tapped into the CAN wires instead of using a proper interface module.
  • U0008 can be stored as a history code after a low battery or jump start, so always check battery voltage and clear codes before beginning invasive diagnostics.

Can I keep driving with U0008?

If the vehicle runs and drives normally with only a warning light, you can drive it to a repair shop, but avoid long trips and be aware that other systems may fail unexpectedly. If multiple warning lights are on or the engine will not start, 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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