U0039: Vehicle Communication Bus B (+) Open

What it means

Bus B high-side line is open — broken wire or disconnected. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Bus B uses paired conductors to carry messages between its connected modules; the (+) conductor is the high-side line. U0039 is set when that positive bus line is detected as open, interrupting communication and potentially leaving dependent modules without the messages they need.

Common causes

Most likely first.

1. Wire broken or connector unplugged
2. Gateway module routing CAN messages incorrectly
3. CAN terminating resistor (120 Ω) drifted out of spec

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Record all codes and freeze-frame data, then identify which bus B modules stop responding when the open fault is present.
2. Inspect bus B (+) connectors and harness sections for an unplugged connector, broken wire, pulled pin, or corrosion, giving priority to areas recently disturbed.
3. Use the wiring diagram and scan-tool network status to locate the modules or bus segments that lose communication together.
4. With power isolated as the manufacturer specifies, test continuity along the bus B (+) path and check for an intermittent open while gently moving accessible harness sections.
5. If the conductor is intact, check the gateway path and bus termination against the manufacturer's specifications before condemning a module.

Common fixes

• Reconnect an unplugged bus B connector and secure it properly.
• Repair a broken or chafed bus B (+) wire and restore sound terminal contact.
• Replace damaged connector terminals or repair corrosion that interrupts the positive line.
• Repair gateway routing or replace a terminating resistor only if network tests confirm that fault.

Can I keep driving?

Avoid driving if the lost bus B communication disables steering, braking, stability control, or another essential function; arrange recovery instead. Even if the vehicle moves normally, limp mode or loss of connected functions can occur while driving.