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DTC codes / U00xx · Network wiring and bus faults / U0038

U0038Vehicle Communication Bus B (Performance)

U0038 is a generic network communication code indicating that the vehicle's computers cannot reliably exchange data on Communication Bus B, which is typically the medium-speed bus used for body and convenience modules. The computer detected a performance fault on that bus, meaning messages are corrupted, delayed, or missing rather than the bus being completely dead.

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

Symptoms you may notice

  • Multiple warning lights illuminated at once, often including ABS, airbag, and traction control
  • Intermittent operation of body accessories such as power windows, door locks, or interior lighting
  • The engine may start and run normally, or may crank without starting depending on which modules are affected
  • A stored U0038 code accompanied by other U-codes in multiple modules
  • Sometimes no symptoms besides the warning light, especially if the fault is intermittent
  • Erratic gauge behavior or flickering instrument cluster displays

Common causes, most likely first

  1. Damaged or chafed wiring on the Bus B signal or return circuit, often near a door hinge or under the dash
  2. Corroded or loose connector pins at a body control module, gateway, or inline harness connector
  3. A failing module on Bus B that is pulling the bus voltage out of specification
  4. Water intrusion into a connector or module causing intermittent shorting
  5. Aftermarket accessories improperly tapped into the bus wiring
  6. A faulty termination resistor or splice pack on the bus
  7. Low battery voltage or a weak ground causing modules to drop off the bus intermittently

How to diagnose and fix it

  1. Verify battery voltage and charge the battery if it is below 12.4 volts, then clear codes and retest
  2. Connect a scan tool and record all stored codes in every module to identify which modules are offline or reporting bus faults
  3. Inspect the Bus B wiring and connectors at each module for chafing, corrosion, and pushed pins, especially at door jambs and under-dash areas
  4. Measure bus voltage and resistance at the diagnostic connector and compare to the manufacturer's specification for Bus B
  5. Disconnect modules one at a time to see if the bus performance fault clears, isolating a module that is dragging the bus down
  6. Repair or replace the damaged section of harness or the faulty connector, then clear codes and perform a full network test
  7. If a module is confirmed faulty, replace and program it according to the manufacturer's procedure, then verify all modules communicate

What most technicians miss on Bus B faults

  • A U0038 on Bus B often sets because a single body module is intermittently shorting the bus, not because the entire bus is failed, so disconnecting modules one at a time is faster than replacing harnesses.
  • Aftermarket alarms, remote starters, and phone chargers tapped into the bus wiring are a common cause that owners forget to mention, so always ask about recent electrical work.
  • The code can be stored by a module that is not even on Bus B, because the gateway reports a performance fault when it cannot relay messages between buses.
  • Water leaks into the trunk or footwell can corrode a splice pack on Bus B and cause intermittent faults that only appear in damp weather.
  • A weak ground at a single module can pull the bus reference voltage down and set U0038 without any visible wiring damage.

Can I keep driving with U0038?

The vehicle may be safe to drive short distances if it starts, runs, and brakes normally, but avoid relying on body conveniences and do not tow if the fault affects safety modules. Have the bus diagnosed promptly because intermittent communication faults can escalate to a no-start or disabled safety system.

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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