U1638 – Vehicle Communication Bus Malfunction

The ODBII code U1638: Vehicle Communication Bus Malfunction indicates a problem with the communication between the vehicle’s control modules, which may affect their ability to exchange data properly.

The severity of ODBII code U1638 is moderate.

Common Causes For: U1637 Invalid or Missing Data for Vehicle Speed.

• Faulty OBD-II connector
• Damaged wiring or connectors in the communication bus
• Malfunctioning control modules
• Poor ground connections
• Corroded or loose terminals
• Software issues or bugs in the control modules
• Interference from aftermarket devices
• Low battery voltage or power supply issues

Common misdiagnosis for U1638 U1638: Vehicle Communication Bus Malfunction

• Faulty OBDII connector
• Bad ground connections
• Defective ECM (Engine Control Module)
• Faulty wiring or harness issues
• Malfunctioning CAN bus modules
• Corrupted software or firmware in control modules
• Issues with other communication protocols (e.g., LIN, K-Line)
• Battery or power supply problems
• Faulty sensors or actuators
• Intermittent electrical connections

Common Troubleshooting Steps

The OBD-II code U1638 indicates a “Vehicle Communication Bus Malfunction.” This code typically points to issues with the communication between various control modules in the vehicle. Here are some common troubleshooting steps to diagnose and resolve the U1638 code:

• Check for Additional Codes: Use an OBD-II scanner to check for any other trouble codes that may be present. Addressing related codes can help pinpoint the issue.
• Inspect the Wiring and Connectors: **Visual Inspection: Check the wiring harnesses and connectors associated with the communication bus for any signs of damage, corrosion, or loose connections.
• **Repair or Replace: Repair any damaged wires or connectors and ensure all connections are secure.
• Check the Communication Bus: **CAN Bus Integrity: If your vehicle uses a CAN (Controller Area Network) bus, check the integrity of the bus. Use a multimeter to measure the voltage levels on the CAN high and CAN low wires.
• **Termination Resistors: Ensure that the termination resistors (typically 120 ohms) are present and functioning at both ends of the bus.
• Inspect Control Modules: **Module Functionality: Check if any control modules are malfunctioning. This can include modules like the Engine Control Module (ECM), Transmission Control Module (TCM), or Body Control Module (BCM).
• **Reprogram or Replace: If a specific module is found to be faulty, it may need to be reprogrammed or replaced.
• Check for Short Circuits: Use a multimeter to check for short circuits in the wiring harnesses. A short can disrupt communication between modules.
• Battery and Ground Connections: **Battery Voltage: Ensure the vehicle’s battery is fully charged and in good condition.
• **Ground Connections: Check all ground connections for corrosion or looseness, as poor grounding can affect communication.
• Scan Tool Communication: Use a professional scan tool to check if you can communicate with all modules. If some modules are unresponsive, it may indicate a communication issue.
• Software Updates: Check if there are any software updates available for the vehicle’s control modules. Sometimes, manufacturers release updates to fix communication issues.
• Clear Codes and Test Drive: After performing repairs, clear the trouble codes and take the vehicle for a test drive to see if the code reappears.
• Consult Technical Service Bulletins (TSBs): Look for any TSBs related to the U1638 code for your specific vehicle make and model. Manufacturers may have specific recommendations for addressing this issue.1
• Professional Diagnosis: If the issue persists after performing the above steps, consider seeking help from a professional mechanic or dealership with experience in diagnosing communication issues. By following these steps, you should be able to identify and resolve the underlying cause of the U1638 code.