U0108

Lost Communication With Alternative Fuel Control Module

U0108 is a generic OBD-II network diagnostic trouble code: Lost Communication With Alternative Fuel Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0108
Group
Network
System
Network
Severity
Warning (MIL on, possible limp mode)
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What U0108 means

U0108 is stored when the PCM or network master module detects that communication with the alternative fuel control module (AFCM) has been interrupted. The AFCM is found on bi-fuel vehicles equipped with compressed natural gas (CNG), liquefied petroleum gas (LPG), or hydrogen fuel systems alongside a conventional gasoline drivetrain. The AFCM manages fuel switching, alternative fuel injector control, tank pressure monitoring, and safety shutoff functions, communicating fuel availability and status to the PCM over the CAN bus.

When U0108 is active, the PCM cannot receive alternative fuel system status, which may disable the ability to switch to or from the alternative fuel. Depending on system design and current operating fuel, the vehicle may continue on its active fuel in a degraded diagnostic state, or it may be forced onto gasoline if the AFCM controls alternative fuel injectors directly. Safety-related alternative fuel shutoffs may also default to the closed position.

Diagnosis should include verifying the AFCM power supply, checking the CAN bus integrity at the module, and inspecting connectors in the often remote locations where AFCMs are mounted (commonly near the trunk or under-body CNG/LPG tank).

Common causes

Most-frequently reported root causes when U0108 is logged.

  • 1
    Open or short in the CAN bus wiring between the AFCM and the main network.
  • 2
    Blown supply fuse for the alternative fuel control module.
  • 3
    Corroded AFCM connector exposed to under-body moisture or fuel vapor.
  • 4
    Failed AFCM internal CAN transceiver or processor.
  • 5
    Heat or vibration damage to the AFCM in its mounting location.
  • 6
    Loose or corroded AFCM ground connection.
  • 7
    Aftermarket CNG or LPG kit installation with incompatible network integration.

Symptoms drivers notice

Alternative fuel system unavailable; vehicle may be forced onto gasoline only.
MIL illuminated; alternative fuel system warning light may be active.
Scan tool cannot communicate with the AFCM.
Alternative fuel gauge or status display may read incorrectly or go blank.
Possible additional fault codes for alternative fuel pressure or injector circuits.

How to diagnose U0108

A typical diagnostic flow when this code is present.

  1. 1
    Scan all modules and record all DTCs to identify whether the fault is isolated to the AFCM.
  2. 2
    Verify the AFCM power supply fuse and check voltage and ground at the module connector.
  3. 3
    Inspect the AFCM connector and harness, paying attention to under-body routing for damage or corrosion.
  4. 4
    Test CAN bus signal quality at the AFCM connector with an oscilloscope.
  5. 5
    Attempt to establish scan tool communication with the AFCM via a module scan.
  6. 6
    Clear DTCs, cycle ignition, and recheck to determine if the fault is intermittent or persistent.
  7. 7
    Replace the AFCM only after confirming power, ground, and bus integrity are satisfactory.

Related network codes

Frequently asked questions

Will U0108 prevent the engine from starting on gasoline?

Typically no. If the vehicle can operate on gasoline independently of the AFCM, it should start and run on gasoline in the fault state.

Is U0108 common on factory bi-fuel vehicles?

U0108 is more commonly encountered on aftermarket CNG or LPG conversions where the AFCM integration with the OEM network is not always factory-standard.

Are there safety concerns with U0108 on CNG vehicles?

Yes. The AFCM controls the alternative fuel shutoff solenoid. If the module is unreachable, confirm that the safety shutoff defaults to the closed (safe) position before driving.

Can U0108 be caused by a discharged auxiliary battery on a bi-fuel vehicle?

If the AFCM has a dedicated power supply that discharged while the main vehicle battery remained charged, the AFCM could drop off the network and generate U0108.

Disabling U0108 in software

RaceTune can permanently disable U0108 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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