U0108
Lost Communication with Alternative Fuel Control Module
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Code U0108 indicates that the vehicle's control network has lost communication with the Alternative Fuel Control Module. This module manages alternative fuel systems such as compressed natural gas (CNG), liquefied petroleum gas (LPG/propane), or flex-fuel (E85 ethanol) systems. It controls fuel switching, injection timing, and fuel system monitoring for the alternative fuel type.
When this communication is lost, the vehicle will typically default to running on its primary fuel (usually gasoline). You can generally continue driving safely on the primary fuel, but the alternative fuel system will be unavailable. This means you lose the cost savings and environmental benefits of the alternative fuel until the issue is resolved.
This code is most common on vehicles with aftermarket alternative fuel conversions, where the wiring and module installation quality can vary. Check the module's power supply and connector first, as aftermarket installations are prone to connection issues. For factory bi-fuel vehicles, the diagnostic process is similar to other lost communication codes: verify power, ground, and CAN bus wiring before condemning the module. A specialist familiar with your specific alternative fuel system may be needed for proper diagnosis and repair.
Severity
Symptoms
- •Vehicle unable to switch between fuel types (e.g., gasoline to CNG or LPG)
- •Alternative fuel system warning light illuminated
- •Reduced engine performance when alternative fuel is selected
- •Higher fuel costs from inability to use the alternative fuel system
- •Check Engine Light on with possible fuel-related secondary codes
- •Rough running or misfires when attempting to use alternative fuel
Likely Causes
The alternative fuel control module and its wiring are often aftermarket additions or are routed in less-protected areas. CAN bus connections to these modules can loosen, corrode, or become damaged over time.
The control module for CNG, LPG, propane, or E85 flex-fuel systems can fail internally. Aftermarket modules are particularly susceptible to vibration damage and thermal cycling failures.
A blown fuse, loose power connection, or corroded ground wire for the alternative fuel module will cause it to power down and stop communicating on the CAN bus.
After vehicle or module software updates, the alternative fuel module may lose its network configuration or become incompatible with the updated CAN bus protocol settings.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Check fuses and power supply to the alternative fuel control module
- Inspect and clean the module connector, looking for corrosion or loose pins
- Verify CAN bus wiring between the module and the main vehicle harness
- Update or reprogram the alternative fuel module software if available
- Replace the module if electrical connections and wiring are confirmed good