B0039: Auxiliary Switch Circuit Open
Diagnostic Trouble Code (DTC) B0039 indicates that the Body Control Module (BCM) or another control module has detected an open circuit in the Auxiliary (AUX) switch circuit. An open circuit means electrical continuity has been interrupted, preventing the control module from receiving the expected switch signal.
Unlike a short to ground or short to battery, an open circuit is usually caused by a broken wire, disconnected connector, damaged switch contacts, or an open fuse. As a result, the AUX switch may stop controlling the intended accessory and the BCM stores DTC B0039.
The exact DTC definition, connector locations, wiring colors, and diagnostic procedures vary between vehicle manufacturers. Always refer to the factory service information for your specific vehicle.
What Does B0039 Mean?
B0039 generally means that the AUX switch signal circuit is electrically open. The BCM continuously monitors the circuit and expects a valid voltage signal whenever the switch is operated. If the circuit remains open, the module cannot detect the correct input and records DTC B0039.
How the AUX Switch Circuit Works
The AUX switch receives power through a protected circuit and communicates with the BCM. When the driver presses the switch, the BCM detects the voltage change and controls the corresponding relay or electronic output. If the circuit is interrupted by a broken wire, disconnected connector, or faulty switch, the signal never reaches the BCM.
Typical AUX Switch Circuit Block Diagram
Figure: Simplified AUX switch circuit. DTC B0039 is typically stored when the signal path is interrupted by an open circuit.
Common Symptoms of B0039
AUX Switch Does Not Function
The switch may not activate the intended accessory because the BCM cannot receive the switch input.
Accessory Inoperative
The connected device, such as auxiliary lighting or another electrical accessory, may remain OFF regardless of switch position.
Stored Diagnostic Trouble Code
The BCM stores DTC B0039 and may illuminate a warning message if supported by the vehicle.
Intermittent Operation
A loose connector or partially broken wire may cause the AUX switch to work intermittently, especially during vibration or vehicle movement.
Common Causes of DTC B0039
Broken Signal Wire
A broken wire is the most common cause of B0039. Continuous vibration, previous repairs, or physical damage may interrupt the AUX switch signal circuit and prevent communication with the Body Control Module (BCM).
Disconnected Electrical Connector
A loose, unplugged, or partially connected connector can create an open circuit, preventing the BCM from detecting the AUX switch signal.
Faulty AUX Switch
Internal wear, damaged contacts, or mechanical failure inside the AUX switch may interrupt the electrical circuit even though the switch appears to operate normally.
Open Fuse
A blown fuse can interrupt power to the AUX switch circuit. Always determine why the fuse failed before installing a replacement.
Corroded or Damaged Terminals
Corrosion, bent pins, loose terminals, or moisture inside electrical connectors can interrupt current flow and create an open circuit.
Faulty Body Control Module (BCM)
Although uncommon, an internal BCM fault may fail to recognize the AUX switch input after all wiring and components have been verified.
How to Diagnose B0039 Using Live Data
A professional OBD-II scan tool with Live Data capability is one of the most effective tools for diagnosing an open AUX switch circuit.
Step 1 – Read All Diagnostic Trouble Codes
Perform a complete system scan and record all stored, pending, and history DTCs. Related body control codes may provide additional diagnostic information.
Step 2 – Monitor Live Data
Access the Body Control Module (BCM) and monitor parameters similar to:
- AUX Switch Status
- AUX Switch Input
- Switch Request
- Input Voltage
- Battery Voltage
Operate the AUX switch several times while monitoring Live Data. Under normal conditions, the switch status should change between ON and OFF. If the value never changes, the BCM may not be receiving the switch signal because of an open circuit.
Parameter names vary by manufacturer and scan tool. Compare the Live Data values with actual switch operation to determine whether the signal reaches the BCM correctly.
Step 3 – Perform an Active Test
If your scan tool supports Active Tests, command the AUX output ON and OFF. If the accessory operates during the Active Test but not when the physical switch is pressed, the output circuit is likely functioning correctly and the fault is probably located in the switch, wiring, or connectors.
Electrical Testing
After completing the scan tool diagnosis, inspect the AUX switch circuit with a digital multimeter. An open circuit is usually caused by a broken wire, disconnected connector, faulty switch, or poor terminal contact.
Step 1 – Inspect the Fuse
Locate the fuse protecting the AUX switch circuit and check it for continuity. If the fuse is open, identify and repair the root cause before replacing it.
Step 2 – Verify Battery Voltage
Turn the ignition ON and measure the supply voltage at the AUX switch connector. If battery voltage is missing, inspect the fuse, power supply circuit, and related wiring.
Step 3 – Check Circuit Continuity
Disconnect the AUX switch and the BCM connector if required by the service manual. Measure continuity through the signal wire using a digital multimeter. Infinite resistance (OL) usually indicates an open circuit that must be repaired.
Step 4 – Inspect Connectors and Terminals
Carefully inspect connectors for loose terminals, bent pins, corrosion, moisture intrusion, or damaged locking tabs. Repair or replace damaged terminals as necessary.
Step 5 – Test the AUX Switch
Measure switch continuity while pressing and releasing the switch. If the internal contacts do not open and close according to the manufacturer's specifications, replace the AUX switch.
Common Repairs for B0039
- Repair broken or open wiring.
- Reconnect loose electrical connectors.
- Replace damaged connector terminals.
- Replace a faulty AUX switch.
- Repair the power supply circuit if voltage is missing.
- Replace the fuse after correcting the underlying fault.
- Replace or reprogram the BCM only after confirming all external components are functioning correctly.
Common Diagnostic Mistakes
- Replacing the BCM before testing circuit continuity.
- Ignoring loose connectors or partially disconnected terminals.
- Skipping Live Data analysis.
- Not measuring continuity with the circuit disconnected.
- Clearing DTCs before recording Freeze Frame data.
- Assuming all AUX circuits operate the same way across manufacturers.
Most B0039 faults are caused by broken wires, disconnected connectors, or worn switch contacts. A simple continuity test with a digital multimeter can often locate the fault much faster than replacing components based on guesswork.
Related Diagnostic Trouble Codes
- B0037 – Auxiliary Switch Circuit Short to Ground
- B0038 – Auxiliary Switch Circuit Short to Battery
- B0040 – Auxiliary Switch Circuit Performance
These DTCs are closely related because they monitor the same AUX switch circuit but detect different electrical faults. Always diagnose the specific fault indicated by the stored code before replacing any components.
Frequently Asked Questions (FAQ)
Can I continue driving with DTC B0039?
Yes. In most cases, B0039 does not affect engine operation or vehicle safety. However, the auxiliary accessory controlled by the AUX switch may stop working until the fault is repaired.
What usually causes B0039?
The most common causes include a broken wire, disconnected connector, damaged switch contacts, corroded terminals, or an open fuse that interrupts the AUX switch circuit.
Can a blown fuse trigger B0039?
Yes. If the fuse supplying the AUX switch circuit is open, the BCM may no longer receive the expected signal and can store DTC B0039. Always identify the reason the fuse failed before replacing it.
Should I replace the BCM?
No. BCM failure is uncommon. Inspect the fuse, power supply, wiring continuity, connectors, and AUX switch before considering BCM replacement or programming.
Conclusion
Diagnostic Trouble Code B0039 indicates an open circuit in the AUX switch signal path. In most cases, the problem is caused by broken wiring, disconnected connectors, damaged switch contacts, or an interrupted power supply. Using an OBD-II scan tool to monitor Live Data, combined with continuity testing using a digital multimeter, provides the fastest and most accurate diagnosis while helping avoid unnecessary replacement of expensive components.
After completing the repair, clear the DTC, operate the AUX switch several times, verify that the Live Data changes correctly between ON and OFF, then perform a final system scan to confirm that B0039 has been resolved.