EOBD/OBDII U0109 Code: Lost Communication with Fuel Pump Control Module (FPCM)

Understanding the U0109 Diagnostic Trouble Code

The Eobd/obdii U0109 trouble code indicates a communication fault within your vehicle’s network. Specifically, it signals a “Lost Communication with Fuel Pump Control Module (FPCM)”. This means that the FPCM, responsible for regulating the fuel pump’s operation, is not effectively communicating with other control modules in your car, such as the Powertrain Control Module (PCM) or Engine Control Unit (ECU).

In modern vehicles, control modules communicate via a network, often a Controller Area Network bus, known as CAN bus. This network allows different modules to share vital information. When the U0109 code appears, it signifies a disruption in this communication pathway, preventing modules from exchanging data with the FPCM. Without proper communication, the fuel pump may not operate correctly, leading to various drivability issues. This code is applicable across a wide range of vehicle manufacturers and models, including but not limited to models from Dodge, Ram, Chevrolet, Ford, GMC, and VW.

Symptoms of a U0109 Code

When your vehicle logs a U0109 code, you might observe several symptoms, including:

• Malfunction Indicator Light (MIL) illuminated: The check engine light will typically be activated on the dashboard.
• Engine cranks but fails to start: If the FPCM cannot communicate and control the fuel pump, the engine may crank but not receive fuel, preventing it from starting.
• Engine hesitation or lack of power: In cases where the engine does start, but communication is intermittent or degraded, you may experience hesitation during acceleration or a noticeable decrease in engine power.
• Poor engine performance: The engine might run roughly, exhibit misfires, or generally perform below its usual standard due to inconsistent fuel delivery.

Common Causes of U0109

Several factors can lead to a U0109 code. The most frequent causes are related to issues within the CAN bus communication network or the FPCM’s power supply:

• Open circuit in the CAN bus + or – line: A break or disconnection in either of the CAN bus circuit wires can disrupt communication.
• Short circuit to power in CAN bus + or – line: If either CAN bus wire is shorted to a power source, it can interfere with signal transmission.
• Short circuit to ground in CAN bus + or – line: Similarly, a short to ground in the CAN bus wiring can cause communication problems.
• Faulty Fuel Pump Control Module (FPCM): Although less common, the FPCM itself can fail, leading to communication issues and the U0109 code.

Diagnosing and Repairing the U0109 Code

Diagnosing a U0109 code requires a systematic approach to pinpoint the communication breakdown. Follow these steps for effective troubleshooting:

1. Initial Checks & Technical Service Bulletins (TSBs): Begin by checking for any relevant Technical Service Bulletins (TSBs) for your specific vehicle make and model. Manufacturers often release TSBs to address known issues, which could save diagnostic time.

2. Scan for Additional Diagnostic Trouble Codes (DTCs): Use an OBD-II scan tool to read all stored DTCs. Prioritize diagnosing any communication-related codes or codes related to battery or ignition issues first. Addressing these foundational problems can sometimes resolve the U0109 code indirectly.

3. Attempt to Communicate with the FPCM: With your scan tool, try to directly communicate with the Fuel Pump Control Module.

   • Successful Communication: If you can communicate with the FPCM and retrieve codes from it, the U0109 code in other modules might be intermittent or a stored historical code.
   • Failed Communication: If you cannot establish communication with the FPCM, the U0109 code is likely active, indicating a current problem.

4. Inspect Fuses and Grounds: The most common cause of communication loss is often a lack of power or ground to the FPCM.

   • Check FPCM Fuses: Locate and inspect all fuses that supply power to the FPCM. Replace any blown fuses.
   • Verify FPCM Grounds: Identify the ground points for the FPCM. Ensure these connections are clean, secure, and free from corrosion. Clean ground connections with a wire brush and baking soda solution if necessary.

5. Wiring and Connector Inspection: If fuses and grounds are intact, inspect the wiring and connectors associated with the FPCM and the CAN bus communication lines.

   • Visual Inspection: Examine the wiring harness for any signs of damage, such as chafing, rubbing, bare wires, burn marks, or melted plastic.
   • Connector Inspection: Disconnect the FPCM connector and carefully inspect the terminals (metal pins) for corrosion (green tint) or burning. Clean terminals with electrical contact cleaner and a plastic bristle brush if needed. Apply dielectric grease after cleaning and drying.

6. Voltage Checks at FPCM Connector: Perform voltage checks at the FPCM connector to confirm power and ground supply. Disconnect the negative battery terminal before disconnecting the FPCM connector for safety.

   • Power Supply Check: Using a digital voltmeter (DVOM), check for battery voltage at each B+ terminal in the FPCM connector with the red lead and a good ground (black lead).
   • Ground Circuit Check: Verify ground integrity by connecting the red lead of your voltmeter to battery positive (B+) and the black lead to each ground terminal in the FPCM connector. You should read battery voltage, confirming a good ground path.

7. CAN Bus Voltage Checks: Check the voltage levels on the CAN bus communication circuits.

   • CAN C+ (HSCAN +) Voltage: With the key in the “ON” position and engine off, connect the red lead of your voltmeter to the CAN C+ wire and the black lead to a good ground. You should see approximately 2.6 volts, with slight fluctuations.
   • CAN C- (HSCAN -) Voltage: Similarly, check the CAN C- wire. You should measure around 2.4 volts, also with slight fluctuations.
   • Non-CAN Bus Systems: If your vehicle uses a different communication protocol, consult the vehicle’s wiring diagrams to determine the correct voltage readings for the communication circuits.

8. Professional Diagnostic Assistance: If all preceding tests are normal and the U0109 code persists, or if communication cannot be established with the FPCM, it may indicate a faulty FPCM. At this point, seeking assistance from a qualified automotive diagnostician is recommended. Replacing the FPCM may require programming or calibration to the vehicle for proper function.

Disclaimer: This information is intended for informational purposes and should not be considered professional repair advice. Always consult with a qualified technician for diagnosis and repair procedures. We are not liable for any actions taken based on this information. All content is copyright protected.