U0039 DTC: Vehicle Communication Bus A Off

What Does U0039 Mean?

Diagnostic Trouble Code (DTC) U0039 indicates that Vehicle Communication Bus A has entered a Bus Off state. This occurs when the CAN controller detects excessive communication errors and automatically disconnects itself from the network to prevent corrupted data from affecting other control modules.

Symptoms

• Check Engine Light illuminated
• ABS, Airbag, or Stability Control warning lights
• Loss of communication with one or more control modules
• Multiple network-related DTCs stored
• Power windows, door locks, or HVAC may stop working
• Instrument cluster warning messages
• Vehicle may enter reduced functionality mode
• Diagnostic scan tool cannot communicate with affected modules

Common Causes

• Severe CAN Bus wiring fault
• Repeated CAN communication errors
• Short circuit or open circuit in the CAN network
• High resistance in CAN wiring
• Water intrusion inside CAN connectors
• Faulty CAN transceiver within a control module
• Defective BCM, PCM, ABS, or another network module
• Low battery voltage or charging system problems

Typical Diagnostic Flow

Battery & Charging System

│

▼

Scan All Control Modules

│

▼

CAN Network CAN High (+) ────────── CAN Low (-) ────────── ✖ BUS OFF ✖

Communication Disabled

▼

Repair CAN Network Fault

How to Diagnose U0039

• Read all communication DTCs using a professional scan tool.
• Inspect the CAN wiring harness for damage, shorts, or open circuits.
• Measure CAN bus resistance (approximately 60 Ω with the ignition OFF).
• Verify CAN High and CAN Low voltages.
• Inspect connectors for corrosion or water intrusion.
• Check battery and charging system voltage.
• Disconnect suspected control modules individually to identify the module causing excessive CAN errors.

Typical CAN Bus Values

• CAN High: approximately 2.5–3.5 V
• CAN Low: approximately 2.5–1.5 V
• Network resistance: approximately 60 Ω (Ignition OFF)
• Actual values vary by vehicle manufacturer.

Estimated Repair Cost

• CAN wiring repair: $80–300
• Connector replacement: $30–150
• Professional CAN network diagnosis: $100–250
• Control module replacement: $250–1,000+

Related DTCs

• U0036
• U0037
• U0038
• U0100
• U0140

Frequently Asked Questions

Can I drive with U0039?

The vehicle may still run, but important electronic systems may stop communicating or become disabled. The fault should be repaired immediately.

What does "Bus Off" mean?

Bus Off means the CAN controller has shut itself down after detecting too many communication errors. This protects the rest of the network from corrupted data.

Will clearing the DTC permanently fix U0039?

No. If the underlying CAN network fault still exists, the controller will enter Bus Off again and the DTC will return.