Broken or chafed wire in the bus B (+) circuit, often due to flexing, corrosion, or rodent damage; Loose or corroded connector pins at a control module or inline harness connector; Failed control module that internally shorts or opens the bus B (+) line; Water intrusion into a connector or module causing corrosion and an open circuit; Aftermarket accessory installation that cut or tapped into the bus wiring incorrectly; Poor ground connection at a module that shares the bus, leading to a perceived open on the positive side

DTC codes / U00xx · Network wiring and bus faults / U0039

U0039Vehicle Communication Bus B (+) Open

Q: What does code U0039 mean?

Vehicle Communication Bus B (+) Open. U0039 indicates that the vehicle's communication bus B positive circuit is open, meaning the control modules that share this bus cannot exchange data because the positive voltage line is broken or disconnected. The computer that monitors the bus detects a loss of voltage on the bus B (+) wire, which prevents normal communication between modules.

U0039 indicates that the vehicle's communication bus B positive circuit is open, meaning the control modules that share this bus cannot exchange data because the positive voltage line is broken or disconnected. The computer that monitors the bus detects a loss of voltage on the bus B (+) wire, which prevents normal communication between modules.

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 airbag lights
The engine may start and run, but other systems such as stability control or transmission shifting may behave erratically
Intermittent or complete failure of dashboard displays, radio, or climate control functions
The vehicle may enter limp mode or refuse to start if the open bus affects critical modules
Often no symptoms besides the warning light if the open circuit is on a non-essential bus segment

Common causes, most likely first

Broken or chafed wire in the bus B (+) circuit, often due to flexing, corrosion, or rodent damage
Loose or corroded connector pins at a control module or inline harness connector
Failed control module that internally shorts or opens the bus B (+) line
Water intrusion into a connector or module causing corrosion and an open circuit
Aftermarket accessory installation that cut or tapped into the bus wiring incorrectly
Poor ground connection at a module that shares the bus, leading to a perceived open on the positive side

How to diagnose and fix it

Use a scan tool to read all stored codes and identify which modules are offline on bus B, then consult the factory wiring diagram for that bus topology
Perform a visual inspection of the bus B (+) wiring and connectors for damage, corrosion, or loose pins, starting at the modules that are not communicating
Measure resistance and voltage on the bus B (+) circuit at each module connector with the key on, comparing to specifications to locate the open segment
Disconnect modules one at a time to see if communication returns, isolating a faulty module that is pulling the bus down
Repair or replace the damaged section of wire, connector, or module as identified, using proper crimping and sealing techniques
Clear all codes and verify that all modules communicate and no codes return after a test drive

Open bus B faults that fool technicians

A single failed module can pull the entire bus B (+) line low, mimicking an open circuit, so disconnecting modules one by one is often faster than chasing wires.
Corrosion inside a connector that looks fine from the outside is a common cause, especially in vehicles with leaks or high humidity, so always check for green or white deposits on pins.
Aftermarket alarms, remote starters, or dash cams tapped into the bus wiring can create an open circuit that only appears after the installation, so ask about recent electrical work.
The code may set intermittently due to a wire that flexes with engine movement or steering, so wiggle-testing the harness while watching live data can reveal the fault.
A poor ground at a module can cause the module to report an open on the bus B (+) side even though the positive wire is intact, so always verify grounds before replacing wiring.

Can I keep driving with U0039?

If the warning light is on but the vehicle drives normally, you can drive cautiously to a repair shop, but avoid long trips or towing. If multiple systems fail or the vehicle enters limp mode, stop driving and have it towed to prevent unsafe operation.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.