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DTC codes / U00xx · Network wiring and bus faults / U0078

U0078Vehicle Communication Bus B (+) Open

U0078 is a generic network communication code indicating that the vehicle's control modules cannot communicate over Communication Bus B because the positive voltage wire of that bus is open. The computer has detected an open circuit on the Bus B (+) line, meaning the expected voltage signal is missing or interrupted, preventing modules from exchanging data.

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

Symptoms you may notice

  • Multiple warning lights illuminated on the dashboard, often including the check engine light, ABS, and traction control
  • The engine may start and run, but other systems like the radio, climate control, or power windows may be inoperative
  • Intermittent or complete failure of dashboard displays and driver information screens
  • The vehicle may enter a limp mode or fail to start if the open bus affects critical modules
  • Often no symptoms besides the warning light if the open bus is on a non-critical network segment

Common causes, most likely first

  1. Broken or chafed wire in the Bus B (+) circuit, often due to flexing, corrosion, or rodent damage
  2. Corroded or loose connector pins at a control module or inline harness connector
  3. Failed control module internally shorting or disconnecting the Bus B (+) line
  4. Water intrusion into a connector or module causing corrosion and an open circuit
  5. Aftermarket accessories improperly spliced into the bus wiring, creating an open or high resistance
  6. Damaged wiring harness from collision, road debris, or improper routing during previous repairs

How to diagnose and fix it

  1. Use a scan tool to read all network codes and identify which modules are offline to narrow the bus segment
  2. Inspect the Bus B (+) wiring and connectors for visible damage, corrosion, or loose pins, starting at the modules that are not communicating
  3. Measure resistance and voltage on the Bus B (+) circuit at each module connector to locate the open point
  4. Perform a wiggle test on harnesses and connectors while monitoring bus voltage to find intermittent opens
  5. Repair or replace the damaged section of wire, connector, or terminal, using proper crimping and sealing techniques
  6. Clear codes and verify communication is restored across all modules with a scan tool
  7. If the open circuit is internal to a module, replace or reprogram that module as needed

Bus B open circuit misdiagnosis traps

  • A common mistake is replacing control modules when the real fault is a simple broken wire or corroded connector pin in the Bus B (+) circuit, so always verify wiring integrity first.
  • The code does not always mean the entire bus is dead; it can be set by a single open segment that isolates one module while the rest of the network continues to function.
  • Aftermarket alarms, remote starters, or audio equipment spliced into the bus wiring are frequent culprits for U0078, especially if the code appeared after installation.
  • Water leaks into the trunk or under the dash can corrode connectors on the Bus B (+) line, causing an open that only appears in damp conditions.
  • Some vehicles use a terminating resistor on the bus; a missing or failed resistor can mimic an open circuit and set U0078 without any broken wires.

Can I keep driving with U0078?

If the vehicle starts and runs normally, you can drive it to a repair shop, but avoid long trips or relying on driver-assist features that may be disabled. If the engine stalls, fails to start, or multiple critical systems are inoperative, stop driving and have the vehicle towed to prevent unsafe operation.

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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