WOMS
P code powertrain Medium severity

P1391 Glow Plug Circuit Low Input (Bank #1)

The OBD2 code P1391 indicates a low input signal from the glow plug circuit on bank #1, suggesting a potential issue with the glow plugs, wiring, or the engine control module (ECM) related to the glow plug system

Definition

The OBD2 code P1391 indicates a low input signal from the glow plug circuit on bank #1, suggesting a potential issue with the glow plugs, wiring, or the engine control module (ECM) related to the glow plug system

Common causes

  • Faulty glow plug
  • Open or shorted wiring in the glow plug circuit
  • Bad glow plug relay
  • Poor electrical connections
  • Faulty engine control module (ECM)
  • Blown fuse in the glow plug circuit
  • Corroded or damaged connectors

Common misdiagnoses

  • Faulty glow plugs
  • Bad glow plug relay
  • Wiring issues or shorts in the glow plug circuit
  • Faulty engine control module (ECM)
  • Incorrectly diagnosed fuel system issues
  • Battery or charging system problems
  • Faulty sensors (e.g., coolant temperature sensor)
  • Injector problems

Troubleshooting steps

  1. 1. Check the Glow Plugs:

    Visual Inspection: Inspect the glow plugs for physical damage or corrosion

  2. 2. Resistance Test

    Use a multimeter to check the resistance of each glow plug. A typical reading should be between 0.5 to 5 ohms. If any glow plug shows infinite resistance, it may be faulty and needs replacement

  3. 3. Inspect the Glow Plug Relay:

    Functionality Check: Test the glow plug relay to ensure it is functioning properly. You can do this by applying power directly to the relay and checking if it activates

  4. 4. Voltage Test

    Use a multimeter to check for voltage at the relay terminals when the ignition is turned on. If there is no voltage, the relay may be faulty or there may be an issue with the wiring

  5. 5. Check Wiring and Connectors:

    Visual Inspection: Look for damaged, frayed, or corroded wiring and connectors in the glow plug circuit

  6. 6. Continuity Test

    Use a multimeter to check for continuity in the wiring from the glow plugs to the relay and the engine control module (ECM). Repair any broken connections

  7. 7. Inspect the Engine Control Module (ECM):

    Check for Faulty ECM: If all other components are functioning correctly, the ECM may be at fault. Check for any software updates or reprogramming that may be needed

  8. 8. Scan for Additional Codes

    Use an OBD-II scanner to check for any other related trouble codes that may provide more insight into the issue

  9. 9. Check for Ground Issues:

    Ground Connections: Ensure that all ground connections related to the glow plug circuit are clean and secure. Poor grounding can lead to low input signals

  10. 10. Test the Power Supply:

    Voltage Check: Ensure that the glow plugs are receiving the correct voltage (typically around 12V) when the ignition is turned on. If not, trace back to the power source

  11. 11. Clear Codes and Test Drive:

    After performing repairs, clear the trouble codes using an OBD-II scanner and take the vehicle for a test drive to see if the code returns

  12. 12. Consult Repair Manual:

    If the issue persists, consult the vehicle’s repair manual for specific troubleshooting steps related to the glow plug circuit for your make and model

  13. 13. Seek Professional Help:

    If you are unable to diagnose or fix the issue, consider seeking help from a professional mechanic who has experience with diesel engines and glow plug systems. By following these steps, you should be able to identify and resolve the issue causing the P1391 code