U0108 Code: Lost Communication With Alternative Fuel Control Module

The U0108 OBD-II trouble code indicates that the vehicle has lost communication with the Alternative Fuel Control Module (AFCM). This communication problem usually occurs through the vehicle's CAN Bus network, which allows control modules to exchange data.

When the PCM/ECM or another control module cannot communicate with the AFCM, the vehicle stores the U0108 code. The problem may be caused by a communication fault, electrical supply issue, wiring damage, or a failed control module.

U0108 Code Definition

DTC U0108: Lost Communication With Alternative Fuel Control Module

U0108 is a network communication trouble code. It is considered a generic OBD-II code, but the exact cause and repair procedure may vary depending on the vehicle manufacturer, model year, and fuel system design.

What Is The Alternative Fuel Control Module (AFCM)?

The Alternative Fuel Control Module is an electronic control unit responsible for monitoring and controlling components related to alternative fuel systems. It communicates with the PCM/ECM and other modules through the CAN Bus network.

The AFCM requires proper power supply, ground connection, and CAN communication lines to operate correctly. If any of these circuits fail, the vehicle may lose communication with the module and store U0108.

Vehicles That May Show U0108 Code

The U0108 code can appear on different manufacturers. The module name and system design can vary between vehicles, so always check the factory wiring diagram for the specific model.

U0108 Ford

On some Ford vehicles, U0108 may occur when the PCM cannot communicate with the Alternative Fuel Control Module. Possible causes include CAN Bus wiring faults, low battery voltage, connector problems, or module communication failure.

U0108 Toyota

Toyota vehicles may display communication faults when a control module on the CAN network becomes unavailable. Diagnosis should include checking power supply, grounds, and CAN communication signals.

U0108 Dodge / Chrysler

On Dodge and Chrysler vehicles, U0108 can be related to communication problems between control modules. Common areas to inspect include CAN wiring, module connectors, and electrical supply circuits.

U0108 Chevrolet / GM

GM vehicles use multiple electronic modules connected through CAN communication. A network fault or voltage problem can cause U0108 along with other U-series communication codes.

Common Symptoms Of U0108

Check Engine Light illumination.
Loss of communication with the Alternative Fuel Control Module.
Additional U-series communication codes stored.
Scanner cannot access the AFCM module.
Reduced engine or fuel system performance.
Limp mode operation in some vehicles.
Starting problems may occur depending on vehicle design.

Common Causes Of U0108

CAN Bus Wiring Problems:
Open circuit, short circuit, or damaged CAN-H and CAN-L communication wires.
Power Supply Failure:
Low battery voltage, blown fuse, faulty relay, or missing module power.
Ground Circuit Problems:
Poor ground connection, corrosion, or damaged ground wiring.
Damaged Connectors:
Loose pins, water intrusion, or corrosion inside connectors.
Faulty Alternative Fuel Control Module:
Internal module failure or programming issues.
Other Network Communication Faults:
A different control module failure can interrupt CAN Bus communication.

Is U0108 Safe To Drive?

The driving risk depends on the vehicle system affected and the symptoms present.

Low Risk: Vehicle operates normally and only a communication code is stored. Short-distance driving may be possible.
Medium Risk: Performance problems or multiple communication codes appear. Diagnosis should be performed soon.
High Risk: The vehicle has starting problems, power loss, or the alternative fuel system stops working. Repair should be completed before normal driving.

Important: Do not replace the Alternative Fuel Control Module before checking the CAN Bus network, power supply, ground circuits, and wiring condition. Many U0108 faults are caused by electrical problems rather than a failed module.

U0108 CAN Bus Communication Diagram

The Alternative Fuel Control Module (AFCM) communicates with the PCM/ECM and other vehicle modules through the CAN Bus network. A communication failure on these lines can trigger the U0108 code.

CAN Bus Signal Flow

PCM / ECM

CAN-H ⇄ CAN-L

Alternative Fuel Control Module (AFCM)

How CAN Communication Works

The PCM/ECM sends and receives digital messages through CAN-H and CAN-L communication lines. The AFCM uses these messages to exchange information about system operation.

CAN-H: High communication line that increases voltage during data transmission.
CAN-L: Low communication line that decreases voltage during data transmission.
Power Supply: Provides operating voltage for the control module.
Ground: Provides a stable electrical reference.

Typical AFCM Wiring Diagram

+12V Battery Power

Ground

CAN-H Communication

CAN-L Communication

Alternative Fuel Control Module

Typical AFCM Pinout

The following pinout is a general example for diagnostic reference. Actual connector pins and wire colors vary by vehicle manufacturer and model year.

Pin	Circuit	Purpose
1	B+	Module Power Supply
2	Ground	Electrical Ground
3	CAN-H	High Speed Communication
4	CAN-L	Low Speed Communication

CAN Bus Fault Examples

Open Circuit

If CAN-H or CAN-L wiring is broken, the AFCM may stop communicating with other modules and U0108 will be stored.

Short Circuit

A short to ground or battery voltage can disturb CAN communication and cause multiple communication codes.

Power Or Ground Failure

If the AFCM loses power or ground, the module will appear offline even if the CAN wiring is correct.

Important: CAN wire colors, connector locations, and pin numbers are different between Ford, Toyota, Dodge, Chevrolet, and other manufacturers. Always use the correct factory wiring diagram before testing.

U0108 Diagnostic Procedure

Before replacing the Alternative Fuel Control Module, a complete diagnostic procedure should be performed. U0108 is a communication code, so the technician must verify network communication, power supply, ground circuits, and module response.

Scanning U0108 With Autel Diagnostic Tool

The following steps are a general example. Menu names may vary depending on the Autel model and vehicle coverage.

Autel Scan Procedure

Connect the Autel scanner to the vehicle's OBD-II port.
Turn the ignition ON and enter:
Diagnostics → Select Vehicle → Auto VIN
Perform a complete vehicle scan:
Control Units → Auto Scan / Health Report
Check all stored communication codes, especially other U-series codes.
Enter the affected module:
Control Unit → Alternative Fuel Control Module (AFCM)
Check:
- Read DTC Information
- Module communication status
- Live Data
- Special Functions (if supported)

Scanning U0108 With Launch Diagnostic Tool

Launch scanners provide similar network diagnostic functions. The exact menu depends on the device and vehicle software.

Launch Scan Procedure

Connect the Launch scanner and select:
Local Diagnosis → Vehicle Brand → Model
Enter:
System Selection → Control Module
Select the Alternative Fuel System or related control module.
Perform:
- Read Fault Codes
- Clear Codes
- Data Stream
- Special Functions (if available)

Live Data Testing For U0108

Live Data helps determine if the module is responding and receiving correct electrical conditions.

Important Parameters To Check

Communication Status:
Check if the AFCM is online or not responding.
Battery Voltage:
Low voltage can cause communication failures between modules.
Module Information:
Verify module identification and response.
Network Status:
Check for missing modules on the CAN network.

Multimeter Testing For U0108

Check Power Supply

Measure voltage at the AFCM power supply pin.
Verify fuses and relays related to the module.
Confirm stable battery voltage.

Check Ground Circuit

Measure voltage drop on the ground circuit.
Inspect ground points for corrosion or loose connections.

Check CAN Bus Resistance

With the ignition OFF and the network inactive, CAN resistance can be checked between CAN-H and CAN-L. Many systems show approximately 60 ohms when termination resistors are correctly connected, but always follow manufacturer specifications.

Communication Fault Diagnosis Logic

No communication with AFCM:
Check power, ground, CAN wiring, and connector condition.
All modules offline:
Suspect a major CAN Bus network problem or power supply issue.
Only AFCM offline:
Check AFCM power supply, ground, wiring, and possible module failure.

Diagnostic Tip: Do not replace the Alternative Fuel Control Module until communication lines, electrical supply, and network integrity have been verified.

U0108 Oscilloscope CAN Bus Testing

An oscilloscope is one of the best tools for checking CAN Bus communication problems. It allows technicians to see the actual communication signals between the PCM/ECM and the Alternative Fuel Control Module.

Oscilloscope Connection Method

Channel A: Connect to CAN-H wire.
Channel B: Connect to CAN-L wire.
Ground: Connect to battery negative.

Normal CAN Bus Waveform

A healthy CAN network shows two opposite signals working together.

CAN-H voltage increases during communication.
CAN-L voltage decreases during communication.
The waveform should be clean and repetitive.
Both communication lines should mirror each other.

CAN Bus Fault Waveforms

No Communication Signal

Flat waveform with no data activity.
Possible causes:
Lost module power supply.
Broken CAN-H or CAN-L wire.
Failed control module.

CAN Line Short To Ground

Signal voltage remains low.
Communication between modules stops.
Possible wiring damage or short circuit.

Distorted CAN Signal

Electrical noise appears on the waveform.
Irregular signal pattern.
Possible causes:
Corrosion in connectors.
Damaged wiring.
Incorrect network resistance.

U0108 Repair Procedure

The repair method depends on the cause found during diagnosis.

CAN Wiring Repair:
Repair open circuits, damaged wires, or short circuits in CAN-H and CAN-L.
Power And Ground Repair:
Restore correct voltage supply and repair poor ground connections.
Connector Repair:
Clean corrosion, repair damaged terminals, and ensure proper connection.
Module Replacement:
If the AFCM is confirmed faulty, replacement and programming may be required.

U0108 Repair Cost

Repair Type	Estimated Cost
CAN Bus wiring repair	$50 - $300
Power or ground repair	$50 - $200
Fuse or relay replacement	$10 - $100
Alternative Fuel Control Module replacement	$300 - $1500+

Common Repair Mistakes

Replacing the AFCM before checking CAN Bus wiring.
Ignoring battery voltage problems.
Replacing parts without checking module communication.
Using incorrect wiring diagrams for another vehicle model.
Not checking other related U-series codes.

Recommended Diagnostic Tools For U0108

Autel diagnostic scanners with full system scan and live data.
Launch diagnostic scanners with network diagnosis functions.
Digital multimeter for power, ground, and resistance testing.
Oscilloscope for CAN waveform analysis.
Vehicle wiring diagrams and service information.

Related DTC Codes

Communication codes that may appear with U0108 or help diagnose CAN Bus problems:

U0100 Code - Lost Communication With ECM/PCM
U0073 Code - Control Module Communication Bus Off
U0101 Code - Lost Communication With TCM
U0121 Code - Lost Communication With ABS Control Module
U0140 Code - Lost Communication With Body Control Module
U0001 Code - High Speed CAN Communication Bus Performance

Frequently Asked Questions

Does U0108 always mean the module is damaged?

No. Many U0108 cases are caused by wiring problems, low voltage, connector faults, or CAN Bus communication issues.

Can a weak battery cause U0108?

Yes. Low battery voltage can interrupt communication between control modules and create multiple network codes.

What is the first step to diagnose U0108?

The first step is scanning all control modules, checking communication status, and verifying power, ground, and CAN wiring before replacing any parts.

Conclusion

The U0108 code requires a complete communication diagnosis, not just replacing the Alternative Fuel Control Module. Checking CAN Bus signals, module power supply, live data, and wiring condition helps identify the real cause and prevents unnecessary repairs.