P1155

What Does Code P1155 Mean?

The diagnostic trouble code P1155 signifies an detected fault within the vehicle’s Alternative Fuel Controller (AFC) or its associated circuit. This code is typically specific to vehicles equipped with systems capable of operating on fuels other than conventional gasoline, such as Ethanol (E85), Compressed Natural Gas (CNG), or Liquefied Petroleum Gas (LPG). The Engine Control Module (ECM), also often referred to as the Powertrain Control Module (PCM), monitors various electrical parameters and operational statuses of all integrated vehicle systems, including the alternative fuel system. When the ECM detects an anomaly in the AFC—such as an out-of-range voltage, a communication error, or an internal self-diagnostic failure—it registers P1155. This indicates that the AFC is either malfunctioning, unable to communicate properly with the ECM, or experiencing a power/ground supply issue. The primary subsystem affected is the vehicle’s ability to utilize or properly manage its alternative fuel source, potentially impacting engine performance, fuel efficiency, or the vehicle’s capacity to operate on the alternative fuel.

Common Symptoms

• Illumination of the Malfunction Indicator Lamp (MIL): The “Check Engine” light will be illuminated on the dashboard.
• Reduced Engine Performance on Alternative Fuel: The engine may run rough, stumble, or lack power when attempting to operate on the alternative fuel.
• Inability to Switch to Alternative Fuel: The vehicle may fail to transition from gasoline to the alternative fuel, or vice-versa.
• Poor Fuel Economy: If the controller is providing incorrect fuel delivery commands, fuel consumption may increase.
• Engine Misfires or Stalling: Erratic fuel delivery or system control can lead to rough idle, misfires, or the engine stalling.
• No Noticeable Symptoms (Intermittent Fault): In some cases, especially with intermittent electrical issues, symptoms may not be immediately apparent, or only occur under specific operating conditions.

What Causes the Code P1155?

• Faulty Alternative Fuel Controller (AFC) Module: The module itself may have failed internally, preventing proper operation or communication.
• Wiring Harness Issues: Damaged, corroded, or open circuits in the power, ground, or communication lines connecting the AFC to the ECM/PCM.
• Poor Electrical Connections: Loose or corroded terminals at the AFC module connector or the ECM/PCM connector.
• Blown Fuse: A fuse protecting the AFC’s power supply circuit may be open.
• Grounding Issues: An inadequate or compromised ground connection for the AFC module.
• ECM/PCM Fault: Though less common, an internal fault within the ECM/PCM preventing it from correctly monitoring or communicating with the AFC can trigger this code.

How to Diagnose and Troubleshoot

Diagnosis of P1155 requires a methodical approach, utilizing an OBD-II scanner and a digital multimeter (DMM).

1. Retrieve and Record Freeze Frame Data: Connect an OBD-II scanner and record all stored freeze frame data. This data captures the engine’s operating conditions (RPM, engine load, coolant temp, etc.) at the precise moment the P1155 code was set, providing crucial context for troubleshooting.
2. Inspect Wiring and Connectors: Perform a thorough visual inspection of the AFC module’s wiring harness and connectors. Look for signs of chafing, cuts, corrosion, or burnt wires. Pay close attention to the integrity of the pins within the connector. Also, inspect the fuse box for any blown fuses related to the alternative fuel system.
3. Check Power and Ground Supply to AFC:
   • With the ignition OFF, locate the AFC module. Disconnect its electrical connector.
   • Using a DMM, check for proper battery voltage (B+) at the power supply pin(s) of the AFC harness connector (refer to vehicle-specific wiring diagrams). This should be approximately 12V with the ignition ON.
   • Check for a good ground connection at the ground pin(s) of the AFC harness connector by setting the DMM to resistance (ohms) and measuring between the ground pin and a known good chassis ground. Resistance should be less than 5 ohms, ideally close to 0 ohms.
   • If voltage or ground is incorrect, trace the respective wiring back to the fuse box, relay, or ground point to identify the open or short circuit.
4. Verify Communication Lines (if applicable): For modules that communicate via a CAN bus or similar serial data line, check the integrity of these circuits. This typically involves checking for continuity between the AFC connector pins and the corresponding ECM/PCM connector pins, and looking for shorts to power or ground. A breakout box can be invaluable here.
5. Monitor Live Data (if available): With the scanner, access live data streams. While many alternative fuel controllers don’t offer extensive PIDs for direct monitoring, look for any parameters related to fuel type selection, fuel pressure (if controlled by the AFC), or system status. An intermittent drop in communication with the AFC may be visible.
6. Test the AFC Module: If power, ground, and communication circuits are verified good, and the code persists, the AFC module itself is highly suspect. Some vehicle manufacturers provide specific diagnostic procedures for testing the module’s internal components or output signals; consult the factory service manual.
7. Perform a Wiggle Test: With the engine running (if possible), gently wiggle the wiring harness and connectors associated with the AFC. If this causes the engine to falter or the code to reappear (if previously cleared), it indicates an intermittent connection issue.

Recommended Repairs and Solutions

The repair strategy for P1155 is highly dependent on the root cause identified during diagnosis.

• Repair or Replace Wiring/Connectors: If visual inspection or DMM tests reveal damaged wiring, corroded terminals, or loose connections, these should be professionally repaired or the affected section of the wiring harness replaced. Ensure all connections are clean, tight, and properly sealed to prevent future issues.
• Replace Blown Fuses: If a blown fuse is found, replace it with a fuse of the correct amperage. It is crucial to investigate why the fuse blew, as it often indicates an underlying short circuit in the protected component or wiring. Without addressing the short, the new fuse will likely blow again.
• Replace the Alternative Fuel Controller (AFC) Module: If all power, ground, and communication circuits to the AFC module are verified to be in perfect working order, and the module itself fails internal self-tests or is suspected to be faulty, replacement is the most likely solution. When replacing an AFC module, always ensure it is the correct part number for the specific vehicle and alternative fuel system. Many modules require programming or a “relearn” procedure with a factory-level scan tool after installation to properly integrate with the vehicle’s ECM/PCM. Failure to perform this step can lead to continued issues or non-operation.
• Address ECM/PCM Faults: In rare instances where the ECM/PCM is definitively diagnosed as the cause (e.g., internal driver failure for a communication line), the ECM/PCM may need to be repaired, reflashed, or replaced. This is a complex repair and typically requires specialized equipment and expertise.
• Mechanics’ Tips: Always consult the vehicle’s specific factory service manual for detailed wiring diagrams, connector pinouts, diagnostic flowcharts, and any module-specific programming procedures. After any repair, clear the DTCs and perform a comprehensive test drive under various operating conditions, including attempting to switch to and operate on the alternative fuel, to ensure the repair is successful and the code does not return. Check for any Technical Service Bulletins (TSBs) related to P1155 for the specific year, make, and model of the vehicle, as manufacturers often release known fixes for common issues.