P1156

What Does Code P1156 Mean?

DTC P1156 indicates a malfunction within the Fuel Select Switch (FSS) circuit. This code is most commonly observed in vehicles equipped with dual fuel tanks (e.g., older trucks, SUVs) or aftermarket alternative fuel systems (e.g., LPG, CNG conversions) that utilize a switch to select between different fuel sources. The Powertrain Control Module (PCM), also known as the Engine Control Module (ECM), monitors the FSS input to determine which fuel tank or source is active, thereby controlling the appropriate fuel pump and potentially adjusting fuel trim strategies for different fuel types.

The PCM sets code P1156 when it detects an electrical signal from the Fuel Select Switch that is outside its programmed parameters. This could manifest as an open circuit (no signal), a short circuit (signal always high or low), an implausible voltage reading for any given switch position, or an inconsistent signal that does not correlate with known switch states. Essentially, the PCM cannot reliably ascertain the driver’s selected fuel source due to an electrical anomaly within the FSS circuit. This directly impacts the fuel delivery subsystem and can prevent the PCM from activating the correct fuel pump or adjusting engine parameters for the specified fuel.

Common Symptoms

• Check Engine Light (CEL) Illumination: The most immediate and common symptom.
• Inability to Select Fuel Tank/Source: The vehicle may be stuck on one fuel tank/source, or the switch may not respond when operated.
• Incorrect Fuel Gauge Reading: The fuel gauge may not accurately reflect the level of the currently selected tank, or it may default to one tank’s reading regardless of selection.
• Engine Performance Issues: If the PCM misinterprets the selected fuel source, it might activate the wrong fuel pump (e.g., attempting to draw from an empty tank) or apply incorrect fuel trim, leading to engine stumbling, misfires, poor acceleration, or even stalling.
• No-Start Condition: In severe cases, if the PCM cannot determine which fuel pump to activate, the engine may crank but fail to start.
• Fuel Pump Inactivity: The fuel pump for the desired tank may not activate when selected.

What Causes the Code P1156?

• Faulty Fuel Select Switch (FSS): The switch itself may have internal electrical failures, worn contacts, or mechanical issues preventing proper signal output.
• Wiring Harness Issues:
  • Open Circuit: A break in the FSS signal wire, power supply wire, or ground wire.
  • Short Circuit: The FSS signal wire shorted to ground or to a voltage source.
  • Chafed or Damaged Wiring: Insulation damage leading to intermittent shorts or opens.
• Corroded or Loose Electrical Connectors: Poor connection at the FSS or at the PCM due to corrosion, bent pins, or inadequate terminal tension.
• Failed Powertrain Control Module (PCM): While rare, an internal fault within the PCM’s input circuit for the FSS could trigger this code. This should be a last resort diagnosis after ruling out all other possibilities.
• External Influence/Modification: Improper installation or malfunction of an aftermarket alternative fuel system’s integration with the vehicle’s original fuel selection logic.

How to Diagnose and Troubleshoot

Diagnosis of P1156 requires a systematic approach, often involving a digital multimeter (DMM) and an OBD-II scanner with live data capabilities. Always consult the vehicle’s specific factory service manual for precise wiring diagrams, connector pinouts, and expected voltage/resistance values.

1. Visual Inspection:
   • Begin by visually inspecting the Fuel Select Switch and its electrical connector for any signs of physical damage, corrosion, or loose connections.
   • Trace the FSS wiring harness as far as possible, looking for frayed wires, chafing against sharp edges, or signs of rodent damage.
   • Check any inline fuses related to the fuel system or FSS, if applicable.
2. OBD-II Scanner Live Data:
   • Connect an OBD-II scanner and access live data parameters. Look for a parameter related to “Fuel Select Switch Status,” “Fuel Tank Selected,” or “FSS Input Voltage.”
   • With the ignition ON (engine off or running, if possible), operate the Fuel Select Switch through all its positions. Observe if the scanner’s live data parameter changes consistently and logically with each switch selection. For voltage-based switches, note the voltage readings in each position and compare them to service manual specifications. An erratic or unchanging reading indicates a fault.
3. Digital Multimeter (DMM) Testing at the FSS Connector:
   • Power and Ground Supply: Disconnect the FSS connector. With the ignition ON, use a DMM to check for appropriate voltage at the power supply terminal (typically 12V or 5V reference voltage) and proper ground at the ground terminal. If either is missing, diagnose the open circuit back to its source (fuse, relay, PCM, chassis ground).
   • FSS Signal Circuit Testing (Disconnected Switch): With the FSS connector still disconnected from the switch, use the DMM to test the internal resistance or continuity of the FSS itself. Cycle the switch through its positions while measuring resistance across the signal and ground terminals. Compare readings to service manual specifications. A switch with infinite resistance where it should have a value, or incorrect/inconsistent resistance, is faulty.
   • FSS Signal Circuit Testing (Connected Switch – Back-probe): If the FSS appears functional in the previous step, back-probe the FSS signal wire at the FSS connector (or PCM connector, if preferred) with the switch connected and ignition ON. Operate the switch and monitor the voltage output. The voltage should change distinctly for each switch position as per service manual specifications (e.g., 0V, 2.5V, 5V for a multi-state switch).
4. Wiring Integrity Testing:
   • If the switch tests good and power/ground are present, but the PCM is not receiving the correct signal, perform continuity tests on the FSS signal wire(s) from the FSS connector to the PCM connector. This verifies the wire is not open.
   • Also, check for any short circuits to ground or to voltage on the signal wire by connecting one DMM lead to the signal wire and the other to chassis ground or battery positive (with the PCM disconnected to prevent damage).
5. PCM Diagnosis: Only after thoroughly verifying the FSS, wiring, and connectors are sound should the PCM be considered as a potential cause. This often requires specialized diagnostic equipment and expertise to confirm a faulty input driver within the PCM.

Recommended Repairs and Solutions

The repair strategy for P1156 is directly dependent on the root cause identified during diagnosis:

• Replace the Fuel Select Switch: If DMM tests confirm the FSS itself is faulty (incorrect internal resistance, inconsistent voltage output, or mechanical failure), replace it with an OEM-quality equivalent. Ensure the replacement switch is appropriate for the specific vehicle and fuel system configuration.
• Repair or Replace Wiring Harness: If damaged or corroded wiring is found, perform precise repairs. For open circuits, solder and heat-shrink connections for durability. For short circuits, isolate and repair the area where the wire is contacting ground or voltage. In cases of extensive damage, replacing the relevant section of the wiring harness may be more effective.
• Clean and Secure Connectors: If corrosion is present on connector terminals, carefully clean them using an electrical contact cleaner and a small brush. Apply a thin layer of dielectric grease to prevent future corrosion and ensure a secure connection. Ensure all terminals have adequate tension for a positive connection.
• Clear DTCs and Verify Repair: After any repair, clear the P1156 code from the PCM using an OBD-II scanner. Perform a test drive, operating the fuel select switch multiple times, to ensure the code does not return and that the system functions correctly. Pay attention to fuel gauge accuracy and engine performance when switching sources.
• PCM Replacement (Rare): If all other components (switch, wiring, connectors) test good, and specialized diagnostics point to an internal PCM fault, replacement of the PCM may be necessary. This often requires programming or flashing the new PCM to the vehicle’s specifications.

Mechanic’s Tips:

• Always disconnect the vehicle’s battery before performing electrical work to prevent accidental shorts or damage to components.
• When dealing with dual fuel tanks, ensure there is sufficient fuel in both tanks when testing to avoid running a pump dry.
• Be mindful that some aftermarket alternative fuel systems might have their own fuel selection logic and control modules that interface with the vehicle’s original system. Any issues with their integration could also trigger this code.
• Always cross-reference your findings with the specific vehicle’s service manual data, as pinouts and expected values can vary significantly between manufacturers and models.