Diagnostic Trouble Code
U0108
Lost Communication with Alternate Fuel Control Module (AFCM)
Technical Description U0108
Lost Communication with Alternate Fuel Control Module (AFCM)
What Does This Mean U0108?
This is a generic communication system diagnostic trouble code that applies to most vehicle makes and models, including but not limited to Dodge, Ram, Chevrolet, Ford, GMC, and VW. This code means that the Alternate Fuel Control Module (AFCM) and other control modules on the vehicle are not communicating with each other.
The circuit most often used to communicate with is known as Controller Area Network bus communications, or simply put, CAN bus. Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to get information from the vehicle, depending on which circuit is affected.
The AFCM operates the fuel pump based upon information received from the Powertrain Control Module (PCM). If no information is received, no fuel pump operation will occur. In compressed natural gas (CNG), liquid natural gas (LNG) or propane applications, the AFCM controls the operation of the shutoff solenoids for the tanks that contain each of these alternate fuels. The AFCM can also be called a Smart Relay Module, and in these applications can also be responsible for providing fuel level information to the instrument cluster.
Troubleshooting steps may vary depending upon manufacturer, type of communications system, number of wires and wire colors in the communication system.
The circuit most often used to communicate with is known as Controller Area Network bus communications, or simply put, CAN bus. Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to get information from the vehicle, depending on which circuit is affected.
The AFCM operates the fuel pump based upon information received from the Powertrain Control Module (PCM). If no information is received, no fuel pump operation will occur. In compressed natural gas (CNG), liquid natural gas (LNG) or propane applications, the AFCM controls the operation of the shutoff solenoids for the tanks that contain each of these alternate fuels. The AFCM can also be called a Smart Relay Module, and in these applications can also be responsible for providing fuel level information to the instrument cluster.
Troubleshooting steps may vary depending upon manufacturer, type of communications system, number of wires and wire colors in the communication system.
Symptoms U0108
1
Malfunction Indicator Light (MIL) On
2
Vehicle will crank but not run
3
If the engine does run: Hesitation; Lack of power; Poor performance; Misfiring cylinders (runs rough)
4
Fuel level gauge reads inaccurately
Possible Causes U0108
1
Open in the CAN bus + circuit
2
Open in the CAN bus - circuit
3
Short to power in either CAN bus circuit
4
Short to ground in either CAN bus circuit
5
Rarely - faulty control module
Solutions & Repair Steps U0108
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known issue with a known fix put out by the manufacturer and can save you time and money during diagnosis.
First, note if there are any other diagnostic fault codes. If any of them are bus communication related, or battery / ignition related, diagnose them first. Misdiagnosis has been known to occur if you diagnose the U0108 code before any of the basic codes have been thoroughly diagnosed and dismissed.
If your scan tool can access fault codes and the only one you get from other modules is the U0108, try to communicate with the AFCM module. If you can access codes from the AFCM module, then the U0108 code is either intermittent or a memory code. If unable to communicate with the GPCM module, then the U0108 code that the other modules are setting is active, and the problem is there now.
The most common failure is loss of power or ground.
Check all fuses that power up the AFCM module on this vehicle. Check all grounds for the AFCM module. Locate where the ground attaching points are on the vehicle and make sure that these connections are clean and tight. If you have to, take them off, get a small wire bristle brush and baking soda/water solution and clean each one, both the connector and where it connects.
If any repairs were made, clear the diagnostic trouble codes from all the modules that set the code in memory, and see if the U0108 code returns or if you are able to communicate with the AFCM module. If the code does not return or communication is re-established with the AFCM module, then the fuses/connections were most likely your problem.
If the code returns, locate the CAN bus communication connections on your particular vehicle, most importantly the AFCM module connector. Disconnect the negative battery cable before unplugging the connector at the AFCM module. Once located, visually inspect the connectors and wiring. Look for scraping, rubbing, bare wires, burn spots or melted plastic. Pull the connectors apart and carefully inspect the terminals (the metal parts) inside the connectors. See if they look burned or have a green tint indicating corrosion. Use electrical contact cleaner and a plastic bristle brush if cleaning of the terminals is needed. Let dry and apply dielectric silicone grease where the terminals contact.
Before connecting the connectors back to the AFCM module, make these few voltage checks. You will need to have access to a digital volt-ohmmeter (DVOM). Verify that you have power and ground at the AFCM module. Gain access to a wiring diagram and determine where the main powers and grounds come into the AFCM module. Reconnect the battery before continuing, with the AFCM module still disconnected. Connect the red lead of your voltmeter to each B+ (battery voltage) supply coming into the AFCM module connector and the black lead of your voltmeter to a good ground (if not sure, battery negative always works). You should see a reading of battery voltage. Verify that you have good grounds as well. Hook the red lead of your voltmeter to battery positive (B+) and the black lead to each ground circuit. Once again you should see battery voltage at each connection. If not, repair the power or ground circuit problem.
Next, check the two communication circuits. Locate the CAN C+ (or HSCAN + circuit) and CAN C- (or HSCAN – circuit). With the black lead of your voltmeter connected to a good ground, connect the red lead to CAN C+. With the Key On, Engine Off, you should see about 2.6 volts and fluctuating slightly. Next, connect the red voltmeter lead to the CAN C- circuit. You should see approximately 2.4 volts and fluctuating slightly.
If the manufacturer of your vehicle is not using CAN bus for communication between the PCM and the AFCM, you will need to determine the appropriate voltages for that communication circuit.
If all tests have passed and communication is still not possible, or you were unable to clear the U0108 fault code, the only thing left that can be done is to seek assistance from a trained automotive diagnostician, as this would indicate a failed AFCM module. Some AFCM modules must be programmed, or calibrated to the vehicle in order to be installed correctly.
First, note if there are any other diagnostic fault codes. If any of them are bus communication related, or battery / ignition related, diagnose them first. Misdiagnosis has been known to occur if you diagnose the U0108 code before any of the basic codes have been thoroughly diagnosed and dismissed.
If your scan tool can access fault codes and the only one you get from other modules is the U0108, try to communicate with the AFCM module. If you can access codes from the AFCM module, then the U0108 code is either intermittent or a memory code. If unable to communicate with the GPCM module, then the U0108 code that the other modules are setting is active, and the problem is there now.
The most common failure is loss of power or ground.
Check all fuses that power up the AFCM module on this vehicle. Check all grounds for the AFCM module. Locate where the ground attaching points are on the vehicle and make sure that these connections are clean and tight. If you have to, take them off, get a small wire bristle brush and baking soda/water solution and clean each one, both the connector and where it connects.
If any repairs were made, clear the diagnostic trouble codes from all the modules that set the code in memory, and see if the U0108 code returns or if you are able to communicate with the AFCM module. If the code does not return or communication is re-established with the AFCM module, then the fuses/connections were most likely your problem.
If the code returns, locate the CAN bus communication connections on your particular vehicle, most importantly the AFCM module connector. Disconnect the negative battery cable before unplugging the connector at the AFCM module. Once located, visually inspect the connectors and wiring. Look for scraping, rubbing, bare wires, burn spots or melted plastic. Pull the connectors apart and carefully inspect the terminals (the metal parts) inside the connectors. See if they look burned or have a green tint indicating corrosion. Use electrical contact cleaner and a plastic bristle brush if cleaning of the terminals is needed. Let dry and apply dielectric silicone grease where the terminals contact.
Before connecting the connectors back to the AFCM module, make these few voltage checks. You will need to have access to a digital volt-ohmmeter (DVOM). Verify that you have power and ground at the AFCM module. Gain access to a wiring diagram and determine where the main powers and grounds come into the AFCM module. Reconnect the battery before continuing, with the AFCM module still disconnected. Connect the red lead of your voltmeter to each B+ (battery voltage) supply coming into the AFCM module connector and the black lead of your voltmeter to a good ground (if not sure, battery negative always works). You should see a reading of battery voltage. Verify that you have good grounds as well. Hook the red lead of your voltmeter to battery positive (B+) and the black lead to each ground circuit. Once again you should see battery voltage at each connection. If not, repair the power or ground circuit problem.
Next, check the two communication circuits. Locate the CAN C+ (or HSCAN + circuit) and CAN C- (or HSCAN – circuit). With the black lead of your voltmeter connected to a good ground, connect the red lead to CAN C+. With the Key On, Engine Off, you should see about 2.6 volts and fluctuating slightly. Next, connect the red voltmeter lead to the CAN C- circuit. You should see approximately 2.4 volts and fluctuating slightly.
If the manufacturer of your vehicle is not using CAN bus for communication between the PCM and the AFCM, you will need to determine the appropriate voltages for that communication circuit.
If all tests have passed and communication is still not possible, or you were unable to clear the U0108 fault code, the only thing left that can be done is to seek assistance from a trained automotive diagnostician, as this would indicate a failed AFCM module. Some AFCM modules must be programmed, or calibrated to the vehicle in order to be installed correctly.