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DTC codes / U01xx · Lost communication with a control module / U0108

U0108Lost Communication With Alternative Fuel Control Module

U0108 means the engine control module (ECM) or another primary controller has stopped receiving valid data from the alternative fuel control module over the controller area network (CAN) bus. The computer detected that the module either stopped broadcasting its periodic status messages or the messages are no longer arriving intact within the expected time window.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaLost communication with a control module

Symptoms you may notice

  • Illuminated check engine light or service vehicle soon message, often with no other noticeable change in how the vehicle runs
  • The vehicle may default to gasoline operation and refuse to switch to the alternative fuel, or the fuel gauge for that system may read incorrectly
  • Rough running, stalling, or a no-start condition if the alternative fuel module is the primary fuel controller on a dedicated gaseous-fuel vehicle
  • Other warning lights such as ABS, traction control, or stability control may illuminate because those modules also rely on the same network
  • Intermittent symptoms that appear in damp weather, over rough roads, or after the vehicle sits unused for several days
  • A no-communication condition with a scan tool that cannot read data from the alternative fuel control module at all

Common causes, most likely first

  1. Damaged, corroded, or loose wiring or connector pins in the CAN bus circuit between the alternative fuel control module and the rest of the network
  2. Failed alternative fuel control module, often due to internal power supply or water intrusion into the module housing
  3. Blown fuse or tripped circuit breaker supplying battery power or ignition power to the alternative fuel control module
  4. Poor ground connection at the alternative fuel control module mounting point or engine block ground strap
  5. Open or shorted CAN high or CAN low wire, sometimes caused by rodent damage, chafing, or a previous repair
  6. Another module on the same CAN bus holding the network down and preventing communication, which the ECM may report against the alternative fuel module
  7. Low battery voltage or a failing charging system that causes modules to drop offline intermittently

How to diagnose and fix it

  1. Connect a scan tool and verify whether the alternative fuel control module responds at all; check for other communication codes on the same bus to see if the fault is isolated
  2. Inspect the battery, charging system, and all fuses and relays that feed the alternative fuel control module, including any underhood or body control module fuse boxes
  3. Visually inspect the module connector and the CAN bus wiring for corrosion, pushed-out pins, chafing, or rodent damage, and perform a wiggle test while watching live data
  4. Measure resistance and voltage on the CAN high and CAN low circuits at the module connector to confirm the bus is intact and not shorted to power or ground
  5. Check the module ground circuit with a voltage drop test under load rather than a simple continuity check
  6. If power, ground, and bus wiring are good, substitute a known-good module of the same part number or perform a module communication test with a factory-level scan tool
  7. After any repair, clear the code and perform a full drive cycle that includes the conditions needed to wake the alternative fuel module, then recheck for pending codes

Why the alternative fuel module goes silent

  • A dead battery or a recent jump-start is one of the most common reasons this code appears, because low voltage can cause the alternative fuel module to drop off the bus and set a communication code that persists after the battery is charged.
  • The code does not prove the alternative fuel module is bad; a single failed module elsewhere on the same CAN bus can pull the network down and cause the ECM to report lost communication with several modules, including this one.
  • On bi-fuel vehicles, the alternative fuel module often shares its power feed with the fuel selector switch or the alternative fuel gauge, so a fault in those circuits can mimic a module failure.
  • Water intrusion into the alternative fuel module connector or housing is a frequent cause on vehicles that spend time in wet or dusty environments, and the corrosion is often hidden inside the connector body.
  • A scan tool that can only read generic OBD-II data may not communicate with the alternative fuel module at all, so a no-communication result on a basic tool does not confirm the module is dead.

Can I keep driving with U0108?

You can usually drive the vehicle if it still runs on gasoline, but avoid long trips or heavy loads until the fault is diagnosed because the vehicle may not switch to the alternative fuel and other network-dependent systems could behave unpredictably. If the vehicle will not start or runs poorly, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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