U0108

Code

Generic U — Network/User

Lost Communication With Alternative Fuel Control Module

Brand: Generic

Type: U — Network/User

Views: UK: 14 EN: 23 RU: 11

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Broken, corroded or disconnected wiring/connector to the alternative fuel control module
No power (fused B+) or poor ground at the module
Failed alternative fuel control module
Network (CAN/LIN) bus fault or missing termination
Software/firmware mismatch or module needing reflash
Intermittent connection caused by moisture, vibration or damaged pins

Symptoms

MIL or network/communication warning lamps illuminated
Reduced or no functionality of alternative fuel system (e.g., no switching, no status messages)
Instrument cluster or info center shows 'service alternative fuel system' or 'module not present' messages
Stored U0108 and possibly other U‑codes for modules on same network
Intermittent or permanent loss of data values from the alternative fuel module on a scan tool

What to check

Use a professional scan tool to confirm U0108 and look for related U-codes and freeze frame data
Verify the alternative fuel control module appears in the scan tool's module list or is absent
Check for network bus-off status or high error counters on the scan tool
Inspect fuses and relays supplying the module for continuity and correct voltage
Visually inspect module connector and harness for corrosion, damaged pins, water intrusion, soot, or crushing
Measure battery voltage at module power pin with key ON and engine OFF

Signal parameters

CAN bus recessive voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (key ON, engine OFF)
CAN bus dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active transmission
Expected differential voltage during messaging: ~1.5 - 2.5 V peak
Module supply (B+) at connector: battery voltage (approx. 12 V with key ON, 12.6 V engine off)
Module ground: near 0 V, low resistance to chassis (< 0.2 Ω desirable)
CAN termination resistance: approx. 60 Ω measured between CAN_H and CAN_L at any convenient location

Diagnostic algorithm

Connect a capable scan tool. Record U0108 and any additional U-codes. Note freeze frame and count of occurrences.
Attempt to communicate with the alternative fuel module. If the module isn't present on the network, proceed with wiring and power checks.
Inspect the module connector and harness for water, corrosion, damaged pins, or rodent damage. Repair or replace damaged sections before further testing.
Verify B+ feed and ground at the module connector (measure voltage and ground resistance). Repair blown fuse, bad relay, or poor ground as needed.
With key ON, measure CAN_H and CAN_L at the module connector. Confirm recessive voltages (~2.5 V) and that differential communication occurs while requesting data from other modules.
Check total CAN termination resistance across CAN_H to CAN_L (~60 Ω). If termination is incorrect, locate and repair missing/shorted termination resistors.
Monitor bus traffic with the scan tool or oscilloscope while performing a wiggle test on the harness and connectors to locate intermittent faults.
Isolate suspected sections: disconnect nearby modules or aftermarket devices one at a time (only if safe) to determine if another node is causing bus errors.
If wiring and power/grounds are confirmed good and the module remains non-responsive, check for available software updates/TSBs and attempt module reprogramming per OEM procedures.
Replace the alternative fuel control module only after confirming the network and power supply are correct. Reprogram/configure and perform re-learning procedures as required.
Clear codes and perform a road test or functional test to verify the repair. Re-scan to confirm U0108 does not return.

Likely causes

Open or short in CAN High/Low or LIN signal wires to the module
Module supply fuse or relay blown or poor B+ at module connector
Damaged module connector pins (corrosion, bent, pushed out)
Bus termination resistor missing or shorted
Module powered but software not responding (needs reprogram)

Fault status

⚠️ Status

Lost Communication With Alternative Fuel Control Module — no/invalid messages received from the alternative fuel control module on the vehicle network.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 3.0 hours

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Code

U0108

HYUNDAI U — Network/User

Lost Communication With Alternative Fuel Control Module | CAN ASG Communication

Brand: HYUNDAI

Type: U — Network/User

Views: UK: 8 EN: 17 RU: 8

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Broken, corroded or disconnected wiring/connector to the alternative fuel control module
No power (fused B+) or poor ground at the module
Failed alternative fuel control module
Network (CAN/LIN) bus fault or missing termination
Software/firmware mismatch or module needing reflash
Intermittent connection caused by moisture, vibration or damaged pins

Symptoms

MIL or network/communication warning lamps illuminated
Reduced or no functionality of alternative fuel system (e.g., no switching, no status messages)
Instrument cluster or info center shows 'service alternative fuel system' or 'module not present' messages
Stored U0108 and possibly other U‑codes for modules on same network
Intermittent or permanent loss of data values from the alternative fuel module on a scan tool

What to check

Use a professional scan tool to confirm U0108 and look for related U-codes and freeze frame data
Verify the alternative fuel control module appears in the scan tool's module list or is absent
Check for network bus-off status or high error counters on the scan tool
Inspect fuses and relays supplying the module for continuity and correct voltage
Visually inspect module connector and harness for corrosion, damaged pins, water intrusion, soot, or crushing
Measure battery voltage at module power pin with key ON and engine OFF

Signal parameters

CAN bus recessive voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (key ON, engine OFF)
CAN bus dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active transmission
Expected differential voltage during messaging: ~1.5 - 2.5 V peak
Module supply (B+) at connector: battery voltage (approx. 12 V with key ON, 12.6 V engine off)
Module ground: near 0 V, low resistance to chassis (< 0.2 Ω desirable)
CAN termination resistance: approx. 60 Ω measured between CAN_H and CAN_L at any convenient location

Diagnostic algorithm

Connect a capable scan tool. Record U0108 and any additional U-codes. Note freeze frame and count of occurrences.
Attempt to communicate with the alternative fuel module. If the module isn't present on the network, proceed with wiring and power checks.
Inspect the module connector and harness for water, corrosion, damaged pins, or rodent damage. Repair or replace damaged sections before further testing.
Verify B+ feed and ground at the module connector (measure voltage and ground resistance). Repair blown fuse, bad relay, or poor ground as needed.
With key ON, measure CAN_H and CAN_L at the module connector. Confirm recessive voltages (~2.5 V) and that differential communication occurs while requesting data from other modules.
Check total CAN termination resistance across CAN_H to CAN_L (~60 Ω). If termination is incorrect, locate and repair missing/shorted termination resistors.
Monitor bus traffic with the scan tool or oscilloscope while performing a wiggle test on the harness and connectors to locate intermittent faults.
Isolate suspected sections: disconnect nearby modules or aftermarket devices one at a time (only if safe) to determine if another node is causing bus errors.
If wiring and power/grounds are confirmed good and the module remains non-responsive, check for available software updates/TSBs and attempt module reprogramming per OEM procedures.
Replace the alternative fuel control module only after confirming the network and power supply are correct. Reprogram/configure and perform re-learning procedures as required.
Clear codes and perform a road test or functional test to verify the repair. Re-scan to confirm U0108 does not return.

Likely causes

Open or short in CAN High/Low or LIN signal wires to the module
Module supply fuse or relay blown or poor B+ at module connector
Damaged module connector pins (corrosion, bent, pushed out)
Bus termination resistor missing or shorted
Module powered but software not responding (needs reprogram)

Fault status

⚠️ Status

Lost Communication With Alternative Fuel Control Module — no/invalid messages received from the alternative fuel control module on the vehicle network.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 3.0 hours