B0038: Auxiliary Switch Circuit Short to Battery
Diagnostic Trouble Code (DTC) B0038 indicates that the vehicle has detected a short to battery (B+) in the Auxiliary (AUX) switch circuit. This means the control module receives a voltage that is higher than expected on the switch input, even when the switch should be inactive.
In many modern vehicles, the AUX switch is monitored by the Body Control Module (BCM) rather than directly controlling the accessory. When the BCM detects constant battery voltage on the signal circuit, it stores DTC B0038 and may disable the affected function to protect the electrical system.
The exact definition of B0038, wiring layout, connector locations, and diagnostic procedures vary between manufacturers. Always compare your findings with the factory service manual for the specific vehicle.
What Does B0038 Mean?
B0038 generally means that the AUX switch signal circuit has been shorted to battery voltage (B+). Under normal conditions, the BCM expects the signal voltage to change only when the switch is operated. If battery voltage is present continuously, the module identifies an abnormal electrical condition and stores the fault code.
How the AUX Switch Circuit Works
The AUX switch receives power through a protected circuit. When the switch is pressed, the BCM detects the input signal and controls an output such as a relay or electronic driver. If the signal wire accidentally contacts a battery voltage source, the BCM continuously sees a high-voltage input and sets DTC B0038.
Typical AUX Switch Circuit Block Diagram
Figure: Simplified AUX switch circuit. DTC B0038 is typically stored when the signal circuit is unintentionally connected to battery voltage.
Common Symptoms of B0038
AUX Switch Does Not Operate Normally
The AUX switch may fail to control the accessory because the BCM constantly detects a high-voltage input.
Accessory Remains Active
Depending on the circuit design, the accessory may remain ON continuously or operate unexpectedly.
Stored Diagnostic Trouble Code
The BCM stores DTC B0038 and may illuminate a warning indicator on vehicles equipped with body system monitoring.
Intermittent Electrical Operation
Damaged wiring or loose connectors may cause the fault to appear only under certain driving conditions.
Common Causes of DTC B0038
Short to Battery Voltage
The most common cause of DTC B0038 is a signal wire that has shorted to battery voltage (B+). This causes the BCM to detect a higher-than-expected voltage on the AUX switch input circuit.
Damaged Wiring Harness
A damaged wiring harness may allow the AUX switch signal wire to contact a constant battery power wire. This frequently occurs near sharp metal edges, moving components, or previous repair areas.
Faulty AUX Switch
An internally damaged AUX switch may continuously supply battery voltage to the BCM input, preventing the module from recognizing normal switch operation.
Loose or Corroded Connectors
Loose terminals, corrosion, or moisture inside electrical connectors can create unstable voltage readings and intermittent circuit faults.
Body Control Module (BCM) Failure
Although uncommon, an internal BCM fault may incorrectly interpret the AUX switch input voltage after all external wiring and components have been verified.
How to Diagnose B0038 Using Live Data
A professional OBD-II scan tool capable of reading Live Data is one of the best tools for diagnosing DTC B0038. Live Data allows you to monitor the AUX switch input while operating the switch.
Step 1 – Read All DTCs
Perform a complete vehicle scan and record all stored, pending, and history codes. If multiple body control codes are present, diagnose them in the order recommended by the manufacturer.
Step 2 – Monitor Live Data
Access the Body Control Module (BCM) and monitor parameters such as:
- AUX Switch Status
- AUX Switch Input Voltage
- Switch Request
- Battery Voltage
- BCM Input Status
Operate the AUX switch while observing Live Data. If the switch status remains constantly ON or the input voltage stays at battery voltage regardless of switch position, inspect the signal circuit for a short to battery.
Different scan tools use different parameter names. Compare the switch status with actual switch operation to determine whether the BCM is receiving the correct input signal.
Step 3 – Perform an Active Test
If your scan tool supports Active Tests, command the AUX output ON and OFF. If the accessory operates correctly during
Electrical Testing
After checking Live Data and completing any available Active Tests, inspect the AUX switch circuit using a digital multimeter. Never replace the Body Control Module (BCM) until the power supply, wiring, connectors, and switch have been thoroughly tested.
Step 1 – Check the Fuse
Locate the fuse protecting the AUX switch circuit and verify continuity. If the fuse is open, inspect the circuit for damage before installing a new fuse.
Step 2 – Measure Supply Voltage
With the ignition ON, measure the voltage at the AUX switch connector. Battery voltage should only be present where specified by the manufacturer. If battery voltage appears on the signal wire when it should not, suspect a short to battery (B+).
Step 3 – Inspect the Signal Wire
Disconnect the AUX switch and BCM connectors if required by the service manual. Inspect the signal wire for contact with another battery-powered circuit, damaged insulation, or previous repair work that may have caused the short.
Step 4 – Inspect Connectors
Check all connectors for corrosion, loose terminals, bent pins, moisture intrusion, or signs of overheating. Repair any damaged terminals before continuing with diagnosis.
Step 5 – Test the AUX Switch
Verify the switch operates correctly using a digital multimeter. Compare the measured values with the manufacturer's specifications. Replace the switch if it continuously outputs battery voltage regardless of its position.
Common Repairs for B0038
- Repair wiring shorted to battery voltage (B+).
- Replace damaged wiring or connectors.
- Replace a faulty AUX switch.
- Repair or replace the relay if applicable.
- Replace a blown fuse only after correcting the root cause.
- Reprogram or replace the BCM only after confirming all external components are operating correctly.
Common Diagnostic Mistakes
- Replacing the BCM before testing the wiring.
- Ignoring damaged insulation in the wiring harness.
- Skipping Live Data analysis.
- Not checking connector terminals for corrosion or loose contact.
- Clearing DTCs before recording Freeze Frame information.
- Assuming all manufacturers use the same AUX switch circuit design.
Most B0038 faults are caused by the signal wire contacting a battery voltage source, damaged connectors, or a faulty AUX switch. Verify the signal voltage changes correctly before replacing expensive control modules.
Related Diagnostic Trouble Codes
- B0037 – Auxiliary Switch Circuit Short to Ground
- B0039 – Auxiliary Switch Circuit Open
- B0040 – Auxiliary Switch Circuit Performance
These DTCs are related to the same AUX switch circuit but indicate different electrical faults. Always diagnose the specific fault indicated by the stored code rather than replacing parts based only on similar symptoms.
Frequently Asked Questions (FAQ)
Can I continue driving with DTC B0038?
Yes, in most cases the vehicle remains drivable. However, the affected auxiliary function may not operate correctly or could remain active continuously. The fault should be repaired as soon as possible.
What usually causes B0038?
The most common causes are a signal wire shorted to battery voltage (B+), damaged wiring insulation, a faulty AUX switch, or corroded electrical connectors.
Can a faulty relay cause B0038?
In some vehicle designs, yes. A relay with internal damage or incorrect wiring may feed battery voltage back into the AUX switch circuit. Always verify the circuit layout using the manufacturer's wiring diagram.
Should I replace the BCM immediately?
No. BCM failure is relatively uncommon. Check the fuse, wiring harness, connectors, relay, and AUX switch before considering BCM replacement or reprogramming.
Conclusion
Diagnostic Trouble Code B0038 indicates that the Body Control Module (BCM) has detected an abnormal battery voltage on the AUX switch signal circuit. In most cases, the problem is caused by damaged wiring, a short to battery, poor electrical connections, or a defective AUX switch. A systematic diagnosis using an OBD-II scan tool, Live Data, Active Tests, and basic electrical measurements will help locate the root cause quickly while avoiding unnecessary parts replacement.
After completing the repair, clear the DTC, operate the AUX switch several times, verify the Live Data changes correctly between ON and OFF, and perform a final system scan to confirm that B0038 does not return.