P0231

What Does Code P0231 Mean?

DTC P0231, indicating “Fuel Pump Secondary Circuit Low,” signifies that the Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected a voltage reading on the fuel pump’s secondary control circuit that is consistently below a calibrated threshold or an unexpected ground condition. The “secondary circuit” typically refers to the control side of the fuel pump relay or the feedback circuit from a Fuel Pump Driver Module (FPDM) to the PCM. It is not the primary high-current circuit directly supplying power to the fuel pump motor itself, but rather the signal or feedback circuit the PCM uses to command the fuel pump to operate or to monitor its operational status. When the PCM commands the fuel pump on, it expects a specific voltage range or a particular signal pattern on this circuit. If the actual voltage measured by the PCM on this feedback or control line falls significantly below the expected value, or indicates an open circuit, P0231 is set. This directly affects the vehicle’s fuel delivery subsystem, potentially leading to insufficient fuel pressure or a complete inability to supply fuel to the engine.

Common Symptoms

• Engine No-Start Condition: The most critical symptom, as the fuel pump may not be receiving the command to operate, preventing fuel delivery.
• Engine Cranks, But Does Not Start: The starter motor functions, but without fuel pressure, combustion cannot occur.
• Intermittent Engine Stalling: If the control signal is erratic, the fuel pump may cease operation unexpectedly during driving.
• Reduced Engine Performance: Including hesitation, lack of power, rough running, or misfires due to insufficient or inconsistent fuel supply.
• Illuminated Check Engine Light (MIL): The primary indicator that a fault has been detected by the PCM.
• Fuel Pump Not Priming: No audible “whine” from the fuel pump when the ignition is initially turned to the ON position.

What Causes the Code P0231?

• Faulty Fuel Pump Relay: An internal failure within the relay, such as corroded contacts or a broken coil winding, can cause an improper voltage drop or an open circuit on the control side, leading to a “low” signal detection by the PCM.
• Wiring Harness Issues:
  • An open circuit in the fuel pump secondary control wire (the wire between the PCM and the relay/FPDM).
  • A short to ground in the fuel pump secondary control wire, which would pull the voltage signal low.
  • Excessive resistance in the control circuit wiring due to corrosion, chafing, or damaged strands.
  • Corroded, loose, or damaged terminals/connectors at the PCM, fuel pump relay, FPDM, or fuel pump itself.
• Faulty Fuel Pump Driver Module (FPDM): If the vehicle is equipped with an FPDM, a malfunction within the module that affects its output signal or feedback to the PCM can trigger P0231.
• Faulty Powertrain Control Module (PCM/ECM): Although less common, an internal failure of the PCM’s fuel pump driver circuit or the monitoring circuit that reads the feedback voltage can incorrectly register a low signal.
• Incorrect Aftermarket Component Installation: Poorly installed aftermarket alarms, remote start systems, or other electronic accessories that interfere with or draw excessive current from the fuel pump control circuit.

How to Diagnose and Troubleshoot

Diagnosis of P0231 requires careful electrical testing using a digital multimeter (DMM) and an OBD-II scanner. Always begin with a thorough visual inspection.

1. Verify Code and Freeze Frame Data: Connect an OBD-II scanner and confirm P0231 is present. Check for any related fuel system DTCs (e.g., P0230, P0232, P0233). Analyze freeze frame data to understand engine conditions (RPM, engine temperature, vehicle speed, etc.) when the fault occurred.
2. Visual Inspection:
   • Carefully inspect the entire wiring harness leading to the fuel pump relay, Fuel Pump Driver Module (if present), and the fuel pump connector. Look for signs of chafing, cuts, corrosion, or burnt insulation.
   • Check all associated connectors for bent pins, corrosion, or loose terminals. Pay close attention to the PCM connector, fuel pump relay socket, and FPDM connector.
   • Ensure the fuel pump relay is securely seated in its socket.
3. Fuel Pump Relay Testing:
   • Locate the fuel pump relay. With the key in the ON position (or during cranking), use a DMM to check for proper voltage supply to the relay coil and a good ground.
   • Test for continuity across the switch side of the relay when it’s de-energized, and ensure it opens when energized by a 12V source (using jumper wires if necessary).
   • If possible, swap the fuel pump relay with a known good, identical relay from another non-critical circuit (e.g., horn relay, if compatible) to see if the code clears or the symptom resolves.
   • As a diagnostic step, carefully jumper the power and load terminals of the fuel pump relay socket to bypass PCM control and directly power the fuel pump. Listen for the pump to operate. If the pump runs, the issue is upstream (relay, wiring to relay, or PCM control).
4. Circuit Voltage and Continuity Checks:
   • Identify the secondary control wire: Consult the vehicle’s wiring diagram to identify the specific wire carrying the secondary control signal from the PCM to the fuel pump relay or FPDM, and any feedback wires.
   • Check for Continuity: Disconnect the PCM and the relay/FPDM connectors. Use a DMM to check for continuity on the identified secondary control wire. There should be very low resistance (typically < 5 ohms). High resistance indicates a damaged wire.
   • Check for Shorts: With the PCM and relay/FPDM disconnected, check for a short to ground (continuity between the control wire and chassis ground) and a short to power (continuity between the control wire and a constant 12V source wire). Any short circuit will cause a P0231.
   • Measure Voltage: With the PCM and relay/FPDM connected, turn the ignition to the ON position (engine off). Measure the voltage on the secondary control wire at the relay/FPDM connector. Compare this reading to factory specifications (often battery voltage or a specific reference voltage from the PCM/FPDM). A significantly low reading (e.g., 0V or close to it) confirms the “low circuit” condition.
5. Fuel Pump Driver Module (FPDM) Diagnosis (If Applicable):
   • Check for proper power and ground supply to the FPDM.
   • Using an oscilloscope, verify the command signal from the PCM to the FPDM (often a Pulse Width Modulated – PWM signal).
   • Verify the feedback signal from the FPDM to the PCM. A faulty FPDM can send an incorrect feedback signal.
6. PCM Output Test: If all wiring and external components test good, the issue may be an internal PCM fault. With all connectors properly seated, use a DMM to measure the voltage output directly at the PCM connector pin for the fuel pump control circuit (while back-probing). Compare to specifications. If the PCM is not outputting the correct voltage while commanding the pump on, the PCM may be faulty.

Recommended Repairs and Solutions

Once the diagnostic steps have isolated the root cause, the following repairs are typically performed:

• Repair or Replace Damaged Wiring/Connectors: If visual inspection and continuity/short tests reveal damaged wires or corroded terminals, these should be repaired using appropriate automotive-grade wiring, connectors, heat shrink, and soldering techniques. Ensure all connections are secure and weatherproof.
• Replace Fuel Pump Relay: If the relay was found to be faulty during testing or if swapping it resolved the issue, replace it with a new, OEM-specified relay. Do not use an incorrect relay as it may not be properly rated.
• Replace Fuel Pump Driver Module (FPDM): If diagnosis points to an internal failure of the FPDM, replace it. Note that some FPDMs may require programming or calibration after installation, so consult service information.
• Clean Corroded Terminals: Thoroughly clean any corroded terminals or connector pins using an electrical contact cleaner and a small brush. Apply dielectric grease to prevent future corrosion.
• Address Aftermarket System Interference: If an aftermarket alarm or remote start system is suspected to be interfering with the fuel pump circuit, temporarily disconnect or remove it to see if the problem resolves. Proper reinstallation or professional removal may be necessary.
• Reprogram or Replace Powertrain Control Module (PCM/ECM): This is generally a last resort, only after all other potential causes (wiring, relays, FPDM) have been thoroughly tested and ruled out. PCM replacement often requires specialized programming and calibration tools, typically performed by a dealership or a specialist shop.

After any repair, clear the DTCs with an OBD-II scanner and perform a comprehensive road test under various driving conditions to confirm the fault does not return. Monitor relevant live data (e.g., fuel pump duty cycle, fuel pressure) if available, to ensure proper system operation.