The OBDII code U0108 indicates a loss of communication between the Alternate Fuel Control Module (AFCM) and other control modules in your vehicle, primarily the Powertrain Control Module (PCM). This critical communication breakdown can prevent the engine from starting or lead to performance issues. This article explains the U0108 code, its symptoms, common causes, and diagnostic steps.
Understanding the U0108 Code and CAN Bus Communication
Modern vehicles rely on a network of interconnected control modules that communicate with each other to manage various functions. The primary communication network is the Controller Area Network (CAN) bus. The AFCM, responsible for controlling the fuel pump and fuel delivery in alternative fuel systems (CNG, LNG, propane), relies on this communication to receive instructions from the PCM. When communication is lost, signified by the U0108 code, the AFCM cannot function correctly. This can also affect the instrument cluster’s ability to display accurate fuel levels.
Symptoms of a U0108 Code
A U0108 code can manifest in various ways, including:
- Illuminated Check Engine Light (MIL): The most common indicator.
- Engine Crank, No Start: The engine may crank but fail to start due to lack of fuel delivery.
- Performance Problems: If the engine runs, you might experience hesitation, lack of power, rough idling (misfiring), and poor overall performance.
- Inaccurate Fuel Gauge: The fuel gauge may provide incorrect readings.
Common Causes of a U0108 Code
Several factors can contribute to a U0108 code:
- Wiring Issues: Open circuits, shorts to power or ground in the CAN bus wiring are frequent culprits. Damaged wiring harnesses, loose connections, or corroded terminals can disrupt communication.
- Faulty AFCM: While less common, a malfunctioning AFCM can be the root cause.
- Power and Ground Problems: Issues with the AFCM’s power or ground supply can prevent it from communicating effectively. This includes blown fuses, loose ground connections, or damaged power supply circuits.
Diagnosing and Repairing a U0108 Code
Before starting any diagnostic procedure, consult your vehicle’s Technical Service Bulletins (TSBs) for known issues and solutions related to the U0108 code.
Diagnostic Steps:
- Scan for Other Codes: Check for other Diagnostic Trouble Codes (DTCs), particularly those related to communication or power/ignition systems. Address these first, as they might be contributing to the U0108 code.
- Communicate with the AFCM: If possible, use a scan tool to attempt communication with the AFCM. Successful communication suggests an intermittent problem or a stored code. Failure to communicate indicates an active problem.
- Inspect Power and Ground: Check all fuses and relays related to the AFCM. Inspect ground connections for cleanliness and tightness. Clean and tighten any suspect connections.
- Visual Inspection of Wiring: Examine the CAN bus wiring harness, particularly near the AFCM connector, for damage, corrosion, or loose connections.
- Voltage Checks: Using a Digital Volt-Ohmmeter (DVOM), verify power and ground at the AFCM connector. Check the voltage on the CAN bus communication circuits (CAN C+ and CAN C-) according to manufacturer specifications.
- Professional Help: If all else fails, consult a qualified automotive diagnostician. A failed AFCM might require replacement and programming.
Conclusion
The U0108 code signifies a serious communication problem that can significantly impact your vehicle’s performance. By understanding the potential causes and following the diagnostic steps outlined above, you can effectively troubleshoot and resolve the issue. However, if you are not comfortable performing these procedures, seeking professional help is recommended. Addressing the U0108 code promptly is crucial to restoring your vehicle’s proper functionality and preventing further damage.
