U0108

Universal

Lost Communication With Alternative Fuel Control Module

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The vehicle is not receiving expected communication from the alternative fuel control module. That module helps manage the vehicle’s alternative fuel system, so the fault may affect its availability or operation. A communication code does not by itself prove the module has failed; the cause could also be in its power supply, wiring, connectors, or the vehicle network.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the vehicle stalls, loses power, or behaves unpredictably, stop driving when it is safe to do so and arrange an inspection. If you notice a fuel odor or suspect a leak, move away from the vehicle and seek qualified assistance. Do not open or work on fuel-system components unless qualified.

Common Symptoms

  • The warning light may be on, although this code can be stored without obvious symptoms.
  • The alternative fuel system may be unavailable or may not operate as expected if its control module cannot communicate.
  • Fuel-mode indications or related messages may be missing or unreliable, depending on the vehicle.
  • Starting or drivability problems are possible if the communication loss affects fuel-system operation; these symptoms do not identify the failed part.

Common Causes

  • A loose, damaged, or corroded connector or wiring fault can interrupt communication with the module.
  • A problem with the module’s power supply or ground can prevent it from communicating.
  • A fault elsewhere on the vehicle communication network can disrupt messages to or from the module.
  • The alternative fuel control module may have an internal fault, but this needs testing before replacement.
  • Low or unstable vehicle voltage may interfere with module operation or network communication.

Diagnostic Steps

  1. Check the vehicle owner’s manual for warning-light guidance and note any changes in starting, fuel-system operation, or drivability.
  2. With the vehicle safely parked and switched off, visually inspect accessible wiring and connectors related to the alternative fuel control system for looseness or visible damage. Do not disconnect fuel lines or open fuel-system components.
  3. Check the battery terminals for obvious looseness or corrosion, and address any battery or charging-system warning according to the vehicle manual.
  4. If you have a suitable scan tool, record the codes from all modules before clearing anything; other communication codes can help identify a wider network or voltage issue.
  5. If the fault returns, symptoms are present, or wiring checks require access to fuel-system components, have a qualified technician diagnose it using vehicle-specific service information.

When to Call a Professional

Use a scan tool to check which modules communicate and whether other network codes are present. Then verify the alternative fuel control module’s power, grounds, connectors, and communication wiring using vehicle-specific service information. Confirm the exact system and applicable diagnostic procedure for the vehicle before replacing a module.

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Commonly Asked Questions

Does U0108 prove that the alternative fuel control module needs replacement?

No. It indicates lost communication, not the reason for it. Power, ground, wiring, connectors, network faults, or the module itself need to be checked.

Can the vehicle still be driven with this code?

It depends on how the vehicle responds and whether the alternative fuel system is affected. If it stalls, loses power, or behaves unpredictably, stop driving when safe and arrange an inspection.

Why might there be no noticeable symptoms?

The code can be stored even if the vehicle’s behavior seems normal. The effect depends on the vehicle’s fuel-system design and how it responds when communication is lost.

What is the difference between a module fault and a network or wiring fault?

A module fault is an internal problem in the control unit. A network or wiring fault prevents messages from reaching it or being sent. Testing power, ground, connectors, wiring, and communication helps distinguish these possibilities.

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