U0108 — Data Bus: Alternative Fuel Control Module – No Communication #

Note: The code description and wording may vary depending on the vehicle manufacturer (OEM).

• U0108: Lost Communication With Alternative Fuel Control Module (GM)
• U0108: CAN Communication Error With Alternative Fuel Module (Ford)
• U0108: No Communication With Alternative Fuel Control Module (Chrysler)

Summary of the U0108 Code #

The U0108 diagnostic trouble code (DTC) signals a communication breakdown between the vehicle’s main computer network—known as the Controller Area Network (CAN) bus—and the Alternative Fuel Control Module (AFCM). This module is responsible for overseeing alternative fuel functions, such as compressed natural gas (CNG), liquefied petroleum gas (LPG), or other non-gasoline/diesel fuels. When the main computer (Powertrain Control Module, or PCM) fails to receive signals from the AFCM, it triggers this code. The U0108 code typically points to a wiring, module, or network-related fault, not a direct engine or fuel system failure. Addressing this code is crucial for alternative fuel vehicles to function properly and comply with emissions standards.

Common Symptoms for Code U0108 #

When U0108 is present, you may notice a combination of drivability and warning light symptoms. Since the Alternative Fuel Control Module is offline, the vehicle may default to its primary fuel mode or disable alternative fuel systems altogether. Typical warning indicators and performance changes may arise.

• Check Engine Light (CEL) or Service Engine Soon indicator illuminated
• Alternative Fuel Warning Light active (if equipped)
• Inability to switch to alternative fuel mode
• Loss of power or reduced engine performance
• Fuel system stuck in gasoline/petrol operation
• Possible emissions test failure

Most Probable Causes for Code U0108 #

The U0108 code usually results from a breakdown in electronic communication rather than a mechanical fault. The root causes often relate to wiring, module failure, or network interference. Here is a ranked list of the most common reasons:

1. Open or damaged wiring/connectors leading to the Alternative Fuel Control Module
2. Faulty Alternative Fuel Control Module
3. Poor electrical ground for the AFCM or related circuits
4. Failed or intermittent CAN bus communication (shorts, open circuits, or high resistance)
5. Blown fuse supplying power to the AFCM
6. Corroded or loose connector pins at the module interface
7. Software/firmware update required for module compatibility

Step-by-Step Diagnostic Guide for Code U0108 #

Diagnosing U0108 requires a methodical approach, beginning with simple checks and progressing to network analysis. Always follow OEM repair procedures for your specific vehicle, as steps may vary. Here’s a general diagnostic sequence:

• Step 1: Confirm the code with a professional-grade OBD-II scanner and check for related codes (especially U-codes or P-codes related to CAN bus or powertrain).
• Step 2: Inspect the Alternative Fuel Control Module wiring harness for damage, chafing, or corrosion.
• Step 3: Check all fuses and relays supplying power to the AFCM.
• Step 4: Test for proper ground continuity at the AFCM connector.
• Step 5: Perform a CAN bus integrity test (check resistance between CAN High and CAN Low wires; should be around 60 ohms with ignition off).
• Step 6: Use scan tool data to see if the AFCM is recognized or communicating on the network.
• Step 7: If wiring and power/ground are OK, consider reprogramming or replacing the AFCM as a last resort.

Possible Repairs for Code U0108 #

Solutions for U0108 depend on the underlying cause identified during diagnosis. Address the most likely and cost-effective fixes first, and always verify repairs by clearing the code and performing a road test.

• Repair or replace damaged wiring/connectors to the Alternative Fuel Control Module
• Clean and secure all ground points related to the AFCM
• Replace blown fuses or faulty relays supplying power to the AFCM
• Update or reflash module software (if required by OEM)
• Replace the Alternative Fuel Control Module if confirmed faulty
• Repair or replace sections of the CAN bus harness as needed

Is It Safe to Drive With Code U0108? #

Driving with code U0108 is generally safe, but with limitations. The main risk is the inability to use the alternative fuel system, meaning your vehicle will default to gasoline or diesel only. You may also fail an emissions inspection if your local regulations require alternative fuel operation. If the vehicle exhibits major drivability problems or fails to start, avoid driving until repairs are completed.

Vehicles Most Commonly Affected by Code U0108 #

The U0108 code is most often found on vehicles equipped with OEM alternative fuel systems, including dedicated CNG, LPG, or bi-fuel models. It is more prevalent in fleet or commercial vehicles but can also occur in personal vehicles with factory alternative fuel options.

• GM (General Motors) CNG/LPG trucks and vans
• Ford CNG-capable F-Series, Transit, and E-Series
• Chrysler/Dodge/RAM CNG or LPG commercial vehicles
• Isuzu NPR and N-Series with alternative fuel packages
• Other OEMs offering factory alternative fuel options

Common Mistakes & Best Practices for Code U0108 #

Misdiagnosis can lead to unnecessary parts replacement and increased downtime. Following proven troubleshooting steps and understanding CAN bus principles are key to resolving this code efficiently.

• Overlooking basic wiring faults—always check for physical connector and harness damage first
• Replacing the AFCM without confirming power/ground supply
• Ignoring related CAN bus codes—they often point to broader network problems
• Neglecting to verify module software compatibility after replacement
• Rushing through diagnostics—take time to consult OEM schematics and bulletins

U0108 Quick FAQ #

Here are answers to some common questions about the U0108 code:

• Q: Can I simply clear the U0108 code and keep driving?
  A: Clearing the code without addressing the underlying cause will only provide a temporary fix. The code will return if the communication issue persists.
• Q: Does U0108 mean the alternative fuel system is permanently disabled?
  A: Not permanently, but the system will remain offline until repairs are made. Restoring communication will allow normal alternative fuel operation.