U0108 Lost Communication With Alternative Fuel Control Module (AFCM)

U0108 is a network communication diagnostic trouble code indicating that a vehicle control module has lost communication with the Alternative Fuel Control Module (AFCM).

The important word in this code is “communication.”

U0108 does not directly say that the alternative-fuel system has a mechanical problem, that an injector has failed, or that the AFCM itself is defective. It means the vehicle's control network expected to receive information from the AFCM but communication was interrupted or lost.

This code is particularly relevant on vehicles equipped with alternative-fuel systems such as CNG, LPG, LNG, or other manufacturer-specific gaseous/alternative-fuel configurations. The actual AFCM architecture varies significantly between manufacturers.

What Is the Alternative Fuel Control Module?

The AFCM is an electronic controller responsible for managing or coordinating parts of an alternative-fuel system.

Its exact responsibilities depend on the vehicle.

An AFCM may be involved with functions such as:

• Alternative-fuel injection

• Fuel selection

• Fuel pressure management

• Fuel-system monitoring

• Fuel temperature monitoring

• Fuel tank information

• Solenoid valve control

• Alternative-fuel injector control

• Fuel-system diagnostics

• Communication with the engine-control module

On some vehicles, the alternative-fuel controller is a completely separate module.

On others, some of these functions may be integrated into another control unit.

Therefore, the generic term AFCM should not be interpreted as meaning that every vehicle has the same module, location, wiring, or function.

What Does "Lost Communication" Mean?

The vehicle's electronic modules communicate with each other through a communication network.

Depending on the vehicle, this can involve:

• CAN bus

• CAN-H

• CAN-L

• Gateway modules

• Other manufacturer-specific communication networks

When the expected messages from the AFCM stop arriving, another module can record U0108.

This can happen because:

• The AFCM has lost power

• The AFCM has lost ground

• A communication wire is damaged

• A connector is loose

• A network circuit is shorted

• Another module is interfering with the network

• The AFCM has an internal electronic failure

• System voltage is unstable

• Software or configuration is incorrect

So the code describes a communication failure, not necessarily an AFCM hardware failure.

Why Alternative-Fuel Vehicles Need an AFCM

An alternative-fuel vehicle often has additional components that a conventional gasoline-only vehicle does not need to manage.

For example, a gaseous-fuel system may require monitoring of:

• Fuel pressure

• Tank pressure

• Fuel temperature

• Gas injectors

• Shut-off valves

• Pressure regulators

• Fuel selection

• Leak detection

• Fuel-system status

The engine-control system needs reliable information from these systems.

If communication with the AFCM disappears, the vehicle may no longer have enough information to safely manage the alternative-fuel system.

As a result, the vehicle may switch fuel, disable alternative-fuel operation, or enter a reduced-power strategy depending on the manufacturer's design.

Common Symptoms of U0108

The symptoms depend heavily on the vehicle and alternative-fuel architecture.

Possible symptoms include:

• Check Engine Light

• Alternative-fuel warning light

• Fuel-system warning message

• Vehicle refusing to switch to alternative fuel

• Vehicle switching back to gasoline

• Reduced engine power

• Poor acceleration

• Engine hesitation

• Difficult starting

• Stalling

• Increased fuel consumption

• Limited alternative-fuel operation

• Multiple communication-related warning lights

Some vehicles may continue operating normally on gasoline while alternative-fuel operation is disabled.

This does not mean the problem is unimportant. It can simply mean that the vehicle's fail-safe strategy has isolated the affected fuel system.

The First Question: Can You Communicate With the AFCM?

This is one of the most useful diagnostic questions.

Connect a scan tool capable of accessing the vehicle's alternative-fuel system and determine whether the AFCM responds.

There are several possibilities.

The AFCM communicates normally

If the scan tool can access the AFCM and U0108 is stored as a history or intermittent fault, investigate intermittent communication, voltage instability, connectors and network wiring.

The AFCM cannot be accessed

If every other module communicates but the AFCM is completely unavailable, concentrate on:

• AFCM power supply

• AFCM ground

• Fuses

• Relays

• Connector

• CAN/network wiring

• AFCM itself

Several modules cannot communicate

This points toward a broader network, power or ground problem.

Replacing the AFCM at this stage would be premature.

Common Causes of U0108

Loss of AFCM power

A controller cannot communicate if it is not receiving the power it needs.

Possible causes include:

• Blown fuse

• Failed relay

• Open power wire

• Corroded fuse connection

• Ignition-power failure

• Loose terminal

• Damaged harness

Always verify the power supply under the conditions specified by the manufacturer.

Poor ground

A poor ground can produce symptoms that look like a dead electronic module.

A ground connection may appear physically secure but have excessive resistance under load.

Possible causes include:

• Corrosion

• Loose ground connection

• Damaged ground wire

• Engine/chassis grounding problem

• Water contamination

CAN wiring damage

If the AFCM communicates through CAN, damaged communication wiring can isolate the module.

Possible problems include:

• Open CAN-H

• Open CAN-L

• Short to ground

• Short to voltage

• Excessive resistance

• Damaged connector

• Poor terminal contact

• Harness chafing

The correct test method and expected network values are manufacturer-specific.

Alternative-Fuel Harness Problems

Alternative-fuel wiring can be exposed to conditions that are particularly demanding.

Depending on the installation, the harness may pass through areas exposed to:

• Engine heat

• Vibration

• Moisture

• Fuel-system components

• Road debris

• Abrasion

• Previous conversion work

On vehicles with aftermarket LPG or CNG systems, installation quality becomes particularly important.

A poorly routed harness, incorrectly repaired wire, loose connector or inadequate grounding can produce communication faults that resemble an electronic module failure.

Aftermarket LPG and CNG Systems

This deserves a separate consideration.

A vehicle may have a factory alternative-fuel system or a conversion system installed later.

The diagnostic architecture can be very different between the two.

With an aftermarket system, the controller may communicate with the original vehicle electronics through an interface or dedicated communication connection.

Therefore, if U0108 appears after:

• LPG conversion

• CNG conversion

• Alternative-fuel ECU replacement

• Harness modification

• Engine work

• Battery replacement

• Electrical repair

inspect the installation and its connections before assuming the controller has failed.

Can a Weak Battery Cause U0108?

Yes, low or unstable system voltage can contribute to communication faults.

A weak battery may cause control modules to reset or temporarily disappear from the network.

Check:

• Battery condition

• Battery terminals

• Charging system

• Voltage during cranking

• Main grounds

• Module power supply

However, a low-voltage event should not become a default explanation for a recurring U0108.

If the code returns under stable electrical conditions, continue with network and AFCM diagnosis.

Scan All Modules, Not Just the Engine ECU

A complete vehicle scan is particularly useful with communication codes.

Look for:

• U0108

• Other U-codes

• Low-voltage DTCs

• Fuel-system codes

• Injector-related codes

• Pressure-sensor codes

• Network gateway faults

• ECM communication faults

• Body-control communication faults

The pattern matters.

For example, if the ECM, transmission controller and body module all report communication problems at the same time, the AFCM may not be the root cause.

There could be a common power supply or network problem.

What If Only U0108 Is Present?

If U0108 is the only code and the AFCM cannot be accessed, the investigation becomes more focused.

A logical sequence is:

AFCM power → AFCM ground → fuses/relays → connector → network wiring → network integrity → AFCM

This is much more efficient than immediately replacing the controller.

Check the AFCM Connector

Inspect the connector carefully.

Look for:

• Bent pins

• Corrosion

• Water intrusion

• Loose terminals

• Pushed-back terminals

• Damaged locking mechanism

• Contamination

• Previous repair work

A connector can be physically connected while one terminal has lost proper contact.

This can create an intermittent communication fault that is difficult to reproduce.

Why Continuity Testing Alone Can Mislead You

A communication circuit may pass a simple continuity test and still have a serious problem.

For example, a damaged conductor can have enough continuity for an ohmmeter but develop excessive resistance when the circuit is operating.

Similarly, a weak connector terminal can pass a static test but lose reliable contact due to vibration.

Depending on the manufacturer, proper diagnosis may involve:

• Voltage-drop testing

• Loaded circuit testing

• CAN resistance testing

• Oscilloscope analysis

• CAN waveform inspection

• Network topology testing

Do not apply generic CAN measurements without considering the vehicle's actual network architecture.

Could Another Module Cause U0108?

Yes.

This is one of the more important aspects of communication diagnostics.

A different module connected to the same network can potentially interfere with communication.

For example, an internal short in another controller or a damaged communication circuit can disrupt network traffic.

If multiple modules suddenly report communication failures, search for the common network or power problem rather than replacing multiple controllers.

Can a Fuel System Problem Cause U0108?

A purely mechanical fuel-system problem generally does not directly cause a lost-communication code.

For example, a clogged fuel filter or faulty fuel-pressure regulator does not normally make an AFCM disappear from the communication network.

However, the AFCM may monitor that fuel system and could generate separate fuel-related DTCs if it remains online.

This distinction helps separate:

Fuel-system performance problem

from

Electronic communication problem

Both can occur at the same time, but they should not automatically be treated as the same fault.

Can U0108 Prevent Alternative-Fuel Operation?

Yes.

If the engine-control system cannot reliably communicate with the alternative-fuel controller, the vehicle may disable alternative-fuel operation as a protective strategy.

Depending on the vehicle, it may:

• Remain on gasoline

• Switch from alternative fuel to gasoline

• Prevent fuel switching

• Limit engine torque

• Illuminate a warning

• Enter a fail-safe mode

The exact strategy is manufacturer-specific.

U0108 After an AFCM Replacement

If the AFCM has recently been replaced and U0108 appears afterward, do not immediately assume the replacement module is defective.

Check whether the vehicle requires:

• Programming

• Configuration

• Coding

• Variant configuration

• Software initialization

• Security authorization

• Fuel-system calibration

A replacement control module may communicate incorrectly or fail to operate properly if it has not been configured for the vehicle.

The exact procedure depends on the manufacturer.

U0108 After Engine Repairs

If the code appeared immediately after engine or fuel-system work, inspect everything that was disturbed.

Pay particular attention to:

• Engine harness

• AFCM connector

• Ground connections

• Fuse box

• Battery connections

• CAN wiring

• Fuel-system wiring

A connector that was disconnected during repairs and not fully locked can produce a communication code without any internal AFCM failure.

A Realistic Diagnostic Scenario

Imagine a CNG-equipped vehicle arrives with U0108 and refuses to operate on CNG.

The technician scans the vehicle and discovers that the ECM cannot communicate with the AFCM.

Instead of ordering a replacement controller, the technician checks AFCM power.

There is no ignition voltage at the module.

The fuse is good.

Further testing reveals that the relay supplying the AFCM is not being activated because of a damaged control wire.

The AFCM itself is completely functional.

Replacing the AFCM would have solved nothing.

Another Scenario: Intermittent U0108

Consider a vehicle that operates normally most of the time but occasionally switches from LPG to gasoline and stores U0108.

The AFCM communicates normally when the vehicle is stationary.

The fault appears during engine vibration.

Inspection reveals that the AFCM harness is routed too tightly and one terminal has insufficient contact tension.

When the engine moves, the connection briefly opens.

The communication interruption causes the ECM to record U0108.

This type of fault demonstrates why intermittent communication codes often require physical harness inspection rather than simple code clearing.

Does U0108 Mean the AFCM Is Bad?

No.

An AFCM failure is one possibility, but it should be considered only after the supporting circuits have been checked.

Before condemning the module, verify:

• Correct power supply

• Correct ground

• Fuse and relay operation

• Connector integrity

• CAN/network wiring

• Network integrity

• Absence of external shorts

• Vehicle configuration/software requirements

Only then does an internal AFCM failure become a strong conclusion.

Should You Replace the Alternative-Fuel ECU?

Not solely because U0108 is stored.

A communication DTC is not a parts-replacement instruction.

If the AFCM has proper power and ground, the communication network is healthy, the connector is good and the manufacturer-specific diagnostic procedure identifies an internal controller fault, replacement may be justified.

After replacement, programming or configuration may be necessary.

Safety Considerations With LPG and CNG Systems

Alternative-fuel systems require additional caution during diagnosis.

LPG and CNG systems can contain pressurized fuel.

Do not disconnect or modify fuel-system components simply to investigate a communication code unless the appropriate safety procedure has been followed.

The electrical diagnosis of U0108 should begin with the control module's electrical and communication circuits rather than opening pressurized fuel lines.

For CNG systems in particular, high storage pressure makes improvised fuel-system work especially dangerous.

A Better Diagnostic Order for U0108

A practical diagnostic sequence is:

1. Perform a complete vehicle scan.

Record all DTCs and identify which modules cannot communicate.

2. Determine whether U0108 is current or historical.

An intermittent history code requires a different approach from a permanent communication failure.

3. Attempt direct communication with the AFCM.

Determine whether the module responds to the scan tool.

4. Check battery and charging voltage.

Eliminate system-voltage problems first.

5. Check AFCM fuses and power supplies.

Use the manufacturer's wiring diagram.

6. Check AFCM grounds under load.

Do not rely only on visual inspection.

7. Inspect connectors and harnesses.

Look for corrosion, terminal problems, chafing and previous repair damage.

8. Test the communication network.

Check the appropriate CAN/network circuits according to manufacturer specifications.

9. Investigate other modules if multiple communication codes exist.

A different controller may be affecting the network.

10. Diagnose the AFCM itself.

Only after the external circuits have been proven should internal AFCM failure become the primary suspect.

Why U0108 Can Be Misdiagnosed

Communication codes are particularly vulnerable to incorrect diagnosis because the module named in the code is often blamed immediately.

For U0108, that could mean replacing an AFCM when the real problem is:

• A fuse

• Relay

• Ground

• CAN wire

• Connector

• Battery

• Harness

• Configuration

• Another module

This is why a good diagnostic process follows the communication path instead of simply following the name of the code.

Final Diagnosis

U0108 – Lost Communication With Alternative Fuel Control Module (AFCM) indicates that a control module has lost expected communication with the vehicle's alternative-fuel controller.

The fault can originate from the AFCM, but equally important possibilities include:

• Loss of module power

• Poor ground

• Blown fuse

• Failed relay

• Damaged wiring

• CAN communication fault

• Loose or corroded connector

• Terminal tension problem

• Low system voltage

• Another module disrupting the network

• Software/configuration problems

• Internal AFCM failure

The most effective diagnosis is to first establish whether the AFCM is powered, grounded and capable of communicating. From there, the technician can work through the network and finally evaluate the controller itself.

Because alternative-fuel systems vary significantly between manufacturers—and because LPG/CNG systems can contain pressurized fuel—the manufacturer's wiring diagram, network architecture and safety procedures should be followed rather than applying a universal test method.