U0029: Vehicle Communication Bus A Performance

What it means

Vehicle communication bus A performance is outside the monitored range. The code does not identify a specific cause.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

Bus A carries messages between its assigned vehicle modules, and the control module monitors whether that communication performs within its expected limits. U0029 indicates abnormal bus A performance, but does not identify the cause; check for wiring dropouts, gateway routing problems, and termination faults rather than assuming a particular line is open or shorted.

Common causes

Most likely first.

1. Intermittent wiring fault on bus B
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. Scan all modules and record freeze-frame or event data, looking for bus A communication errors and the modules that drop offline together.
2. Inspect bus A connectors and harness sections for intermittent contact, corrosion, moisture, or chafing; gently move accessible wiring while watching scan-tool communication status.
3. Check power and grounds at modules reporting bus A faults, since unstable module supply can corrupt network traffic.
4. Use a scan tool's live network status or a bus analyzer to correlate communication errors with vehicle movement or electrical loads.
5. Check bus termination and gateway routing against the manufacturer's service information, then isolate suspect nodes using the specified procedure.

Common fixes

• Secure or clean a loose or corroded bus A connection.
• Repair intermittent wiring or a damaged harness section on bus A.
• Replace a terminating resistor only if its measured value is outside the manufacturer's specification.
• Repair gateway routing or replace a disruptive module only after isolation testing confirms the fault.

Can I keep driving?

If communication loss affects only nonessential features, drive cautiously and arrange diagnosis soon. Avoid driving if bus A faults disable safety-critical controls or cause unpredictable operation.