OBD-II trouble code
U0108: Lost Communication With Alternative Fuel Control Module
A module has stopped receiving messages from the alternative fuel control module over the data network. This module manages CNG, LPG, or bi-fuel operation, so a comms loss can affect running on the alternative fuel.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $900
- DIY difficulty
- Advanced DIY
What does U0108 mean?
U0108 is a lost-communication network code: the receiving module (typically the ECM/PCM) is no longer receiving expected messages from the alternative fuel control module on the communication bus. On vehicles that run compressed natural gas (CNG), liquefied petroleum gas (LPG/propane), or a bi-fuel setup, this module manages the alternative-fuel system — fuel selection, pressure, and injection for that fuel. When the module goes silent on the network, U0108 is stored. This differs from an 'invalid data' code (its counterpart in the U04xx range), where the module is still talking but sending implausible values; here the module has effectively dropped off the bus.
Common causes are power or ground loss to the alternative fuel module, damaged or corroded bus wiring and connectors, and — very commonly on these vehicles — problems in the aftermarket or conversion wiring, since many alternative-fuel systems are added or upfitted. An internally failed module or a wider bus fault can also be responsible. Symptoms often include the check engine light, an inability to switch to or run properly on the alternative fuel, and possibly a fallback to gasoline on bi-fuel vehicles. Diagnosis verifies the module's power, ground, and bus connections — paying special attention to conversion wiring — before replacing the module.
Common causes
- Loss of power or ground to the alternative fuel control module
- Damaged, corroded, or loose bus wiring or connectors
- Faulty aftermarket/conversion wiring on upfitted systems
- Internally failed alternative fuel control module
- Blown fuse feeding the module
- A wider communication (bus) fault affecting multiple modules
Symptoms
- Check engine light on
- Unable to switch to or run properly on the alternative fuel
- Fallback to gasoline operation on bi-fuel vehicles
- Reduced power or driveability issues on the alternative fuel
- Companion lost-communication or network codes stored
Diagnostic steps
- 1.Scan all modules and record every code; note companion network codes suggesting a shared bus fault.
- 2.Locate the alternative fuel control module and confirm its power and ground feeds.
- 3.Inspect bus wiring and connectors — pay special attention to aftermarket/conversion harnesses.
- 4.Check the module's fuse and connector for corrosion and secure seating.
- 5.Verify the module appears on the network with a scan tool; confirm whether it responds.
- 6.If power, ground, and wiring are good but the module stays offline, suspect the module and replace/program per procedure.
Repair cost
$100 – $900
Wiring, connector, or fuse repairs are often $100-$400, and conversion-wiring faults are common on these systems. A failed alternative fuel control module with programming is the costlier case. Confirm power, ground, and the (often aftermarket) wiring before replacing the module.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with control module replacement & programming preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.