U0108 – Lost Communication With Alternative Fuel Control Module

System: Network | Standard: ISO/SAE Controlled | Fault type: General

Definition source: SAE J2012/J2012DA (industry standard)

DTC U0108 indicates the vehicle has detected a loss of communication with the Alternative Fuel Control Module over the network. In practical terms, at least one other module expected to exchange messages with that module and determined the messages were missing, invalid, or timed out for long enough to set a fault. Because network layouts, module responsibilities, and “who talks to whom” vary by vehicle, the exact enabling conditions, related symptoms, and recovery behavior can differ. Always confirm the vehicle’s network topology, module locations, fuse assignments, and diagnostic routines using the correct service information before testing or replacing parts. Treat U0108 as a communication problem first, not an automatic confirmation that the alternative fuel system itself is mechanically faulty.

What Does U0108 Mean?

U0108 – Lost Communication With Alternative Fuel Control Module means the networked control system detected that communication with the Alternative Fuel Control Module was lost. Under SAE J2012 DTC structure conventions, this is a network-related fault where a module (or multiple modules) reports it can no longer reliably receive expected data from the Alternative Fuel Control Module. The code does not, by itself, specify whether the cause is a network wiring issue, a connector problem, missing module power/ground, or an internal module fault; it only states that the required communication was not present as expected. Proper diagnosis focuses on verifying network integrity and the module’s ability to power up and participate on the bus.

Quick Reference

• Subsystem: Network communications involving the Alternative Fuel Control Module.
• Common triggers: Module offline, network bus open/short, poor connector contact, missing ignition feed or ground, or network interference causing message timeouts.
• Likely root-cause buckets: Wiring/connector faults; power/ground supply issues; network bus faults; module hardware/software faults (after basics are verified).
• Severity: Varies by vehicle; may cause drivability changes, reduced functionality, or a no-start if the module is required for authorization or fuel management.
• First checks: Scan for companion network/power DTCs, confirm module powers up, inspect fuses/grounds, and check network wiring/connectors for damage or corrosion.
• Common mistakes: Replacing the module or alternative-fuel components before verifying power/ground, connector pin fit, and network bus integrity.

Theory of Operation

On vehicles equipped with an Alternative Fuel Control Module, that module typically exchanges data with other controllers over the vehicle network. Depending on design, it may report status, sensor interpretations, fuel selection state, and diagnostic information, while receiving commands or requests from other modules. Each participating module expects certain messages at defined intervals and with valid formatting; if those messages stop arriving or fail validity checks, the receiving module increments an error counter and may set a “lost communication” DTC.

U0108 is generally set when message timeouts occur long enough to be considered a true loss of communication rather than a brief disturbance. The underlying cause may be the module not powering up, a disrupted network path (open, short, high resistance, poor terminal tension), or bus conditions that prevent reliable communication. The vehicle may enter a fallback strategy that limits certain functions until communication is restored.

Symptoms

• Warning light: Malfunction indicator or other warning lamp illuminated, depending on how the vehicle reports network faults.
• Driveability: Reduced power, altered throttle response, or limited operating modes if alternative-fuel coordination is required.
• Fuel mode: Inability to switch fuel modes or system defaulting to a single mode (varies by vehicle).
• No-start: Engine may crank but not start, or start and stall, if the missing communication is critical for fuel delivery/enable logic.
• Network codes: Additional U-codes stored in multiple modules indicating broader communication disruption.
• Intermittent behavior: Symptoms that come and go with vibration, temperature, or harness movement consistent with an intermittent connection.

Common Causes

• Open, shorted, or high-resistance wiring in the communication network between the main controller(s) and the Alternative Fuel Control Module
• Poor connector pin fit, corrosion, terminal fretting, or partially unseated connectors at the Alternative Fuel Control Module or network junctions
• Loss of module power supply (blown fuse, failed relay, open feed) to the Alternative Fuel Control Module
• Module ground fault (loose ground fastener, ground splice issue, broken ground wire) causing the module to drop off the network
• Network backbone issue affecting multiple modules (damaged twisted-pair, short between network lines, short to power or ground)
• Aftermarket or non-factory electrical additions interfering with network integrity or module power/ground
• Alternative Fuel Control Module internal fault (hardware failure) after power/ground and network integrity are verified
• Software/configuration issue (module not correctly programmed/configured for the vehicle) where applicable by design

Diagnosis Steps

Tools that help: a scan tool capable of full-network module discovery and reading freeze-frame/data logs, a digital multimeter, and backprobing leads. A wiring diagram and connector pinout from the correct service information are essential. If available, a breakout lead or fused jumper can aid circuit checks without damaging terminals.

1. Confirm the DTC and capture context. Record stored and pending codes, freeze-frame (if available for network events), and run a full module scan to see whether the Alternative Fuel Control Module is present on the network and which other modules report “lost communication” codes.
2. Check for higher-priority electrical DTCs first. If there are battery/ignition feed, charging system, or multiple network DTCs across many modules, diagnose those first because they can cause U0108 as a downstream symptom.
3. Attempt direct communication with the Alternative Fuel Control Module using the scan tool. If it will not respond, note whether the scan tool can communicate with other modules normally; this helps separate a single-module issue from a broader network problem.
4. Perform a quick visual inspection focused on fundamentals: module connectors fully seated, harness routing, signs of rubbing/pinching, water intrusion, corrosion, or previous repairs. Correct any obvious issues before deeper testing.
5. Verify power feeds to the Alternative Fuel Control Module. Using the wiring diagram, check all module B+ and ignition-switched feeds at the module connector or designated test points. If a fuse is open, do not replace it repeatedly without locating the cause of the overload or short.
6. Verify module grounds with a voltage-drop test. With the circuit loaded (key on and the system active as applicable), measure voltage drop across each ground path from the module ground pin to the battery negative or specified ground point. Excessive drop indicates resistance in the ground path that can interrupt network operation.
7. Check the communication network physical layer at the module connector. Inspect terminals for spread pins, push-outs, moisture, or corrosion. Repair terminal fitment issues per service procedures; do not “tighten” terminals by improvisation unless the service method allows it.
8. Evaluate network integrity for opens/shorts. With the module disconnected as needed per service information, test for continuity and shorts to power/ground on the network circuits. Look for a short between network lines, a short to ground, or an open that would isolate the module from the rest of the bus.
9. Perform a wiggle test while monitoring. With the scan tool connected and live-data/logging active (network status, module presence, and DTC setting conditions), gently manipulate the harness and connectors near the module and along common pinch points. If communication drops in/out, isolate the exact movement/location that triggers the fault.
10. Recheck after repairs and confirm. Clear DTCs, cycle the ignition as required, and road test or run the enabling conditions (varies by vehicle). Re-scan to verify the Alternative Fuel Control Module consistently appears on the network and U0108 does not return as stored or pending.
11. If all wiring, power, ground, and network checks pass, consider module or configuration. Verify the module is correctly powered, properly grounded, and correctly connected; then follow service information for any module reset, reinitialization, or programming/configuration checks. Replace the module only after verifying external causes are eliminated.

Professional tip: Treat U0108 as a communication integrity problem until proven otherwise. If the Alternative Fuel Control Module is intermittently offline, prioritize voltage-drop testing on its grounds and loaded checks of its power feeds, then correlate the fault with live-data logs during a wiggle test; intermittent power/ground interruptions commonly mimic “network” failures.

Possible Fixes & Repair Costs

Repair costs vary widely because U0108 is a network communication fault and the true cause can range from a simple connection issue to power/ground faults or a module-related problem. Parts availability, diagnostic time, and harness access all affect total labor and repair scope.

• Repair wiring faults: Restore continuity on network communication lines to the Alternative Fuel Control Module by repairing opens, shorts, or high-resistance sections found during testing.
• Clean, secure, and reseat connectors: Address corrosion, moisture intrusion, backed-out terminals, poor pin fit, or loose locks at the module and any inline connectors in the communication path.
• Restore module power and ground: Repair blown fuses, faulty relays, poor ground points, or damaged power/ground wiring that prevents the Alternative Fuel Control Module from operating on the network.
• Correct network integrity issues: Repair wiring problems affecting the shared network (such as shorted harness sections) that disrupt communication with multiple modules.
• Update or reconfigure software: Perform software updates, configuration, or setup procedures when service information indicates communication faults can be caused by calibration or configuration issues.
• Replace the Alternative Fuel Control Module: Replace only after verifying power/ground and network circuits are correct and the module fails to communicate under known-good conditions.

Can I Still Drive With U0108?

Sometimes the vehicle may still be drivable with U0108, but behavior varies by vehicle and by how the alternative-fuel system is integrated with engine management. If you have a no-start, stalling, reduced power, warnings related to propulsion control, or other safety-related alerts, avoid driving and diagnose the communication, power/ground, and network circuits first. Even when it seems to drive normally, a lost-communication fault can become intermittent and leave you without alternative-fuel operation or cause unexpected drivability changes.

What Happens If You Ignore U0108?

Ignoring U0108 can lead to recurring warning lights, loss of alternative-fuel functionality, failsafe operation, and intermittent drivability issues as network communication degrades. If the root cause is a wiring or connection problem, it may worsen over time and potentially create additional network faults or multiple-module communication codes, increasing diagnostic complexity and repair time.

Last updated: March 14, 2026

Vehicles Commonly Affected by U0108

• Vehicles equipped with an alternative-fuel system that uses a dedicated Alternative Fuel Control Module.
• Platforms with multiple networked control modules where propulsion-related modules share communication lines.
• Vehicles with recent electrical repairs involving harness routing, connector disturbance, or underhood service near module wiring.
• Vehicles exposed to harsh environments that increase the likelihood of connector corrosion or moisture intrusion.
• High-mileage vehicles where vibration and thermal cycling can reduce terminal tension and increase resistance.
• Vehicles with prior battery or power issues that may cause module resets and communication dropouts.
• Vehicles with aftermarket electrical additions that may affect power/ground quality or harness integrity (varies by vehicle).
• Vehicles with network wiring routed through high-stress areas such as near heat sources or moving components (varies by vehicle design).

For best results, treat U0108 as a communication fault first: prove the module’s power/ground and the network wiring are healthy, then only replace parts that fail objective tests confirmed against service information.