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P code powertrain High severity

P1621 Control Module Long Term Memory Performance/ Immobilizer Code Words Do Not Match

The OBD2 code P1621 indicates that there is a mismatch between the control module's long-term memory and the immobilizer code words, which can prevent the vehicle from starting

Definition

The OBD2 code P1621 indicates that there is a mismatch between the control module's long-term memory and the immobilizer code words, which can prevent the vehicle from starting

Common causes

  • Faulty Engine Control Module (ECM)
  • Corrupted or missing software in the ECM
  • Incorrectly programmed or mismatched key or immobilizer
  • Faulty wiring or connections between the ECM and immobilizer
  • Battery voltage issues or low battery
  • Faulty immobilizer system components
  • Aftermarket modifications or installations affecting the ECM
  • Poor ground connections or electrical shorts

Common misdiagnoses

  • Faulty Engine Control Module (ECM)
  • Bad Key or Key Fob
  • Ignition Switch Issues
  • Wiring or Connector Problems
  • Faulty Immobilizer System
  • Corrupted Software or Firmware
  • Battery Voltage Issues
  • Faulty Security System Components

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other trouble codes that may be present. Addressing related codes can help in diagnosing the issue more effectively

  2. 2. Inspect Battery and Connections

    Ensure that the vehicle's battery is in good condition and fully charged. Check for any loose or corroded battery terminals and connections, as poor power supply can affect the ECM's performance

  3. 3. Reset the Code

    Clear the trouble code using an OBD-II scanner and see if it returns after a short drive. If the code reappears, further investigation is needed

  4. 4. Check Wiring and Connectors

    Inspect the wiring and connectors related to the ECM and immobilizer system for any signs of damage, corrosion, or loose connections. Repair or replace any faulty components

  5. 5. Test the ECM

    If the wiring and connections are in good condition, the ECM itself may be faulty. Testing the ECM with a specialized tool or replacing it with a known good unit can help determine if it is the source of the problem

  6. 6. Check the Immobilizer System

    Ensure that the immobilizer system is functioning correctly. This may involve checking the key fob, key transponder, and the immobilizer module. If the key is not recognized, it may need to be reprogrammed or replaced

  7. 7. Reprogram the ECM/Immobilizer

    If the ECM or immobilizer has been replaced, it may need to be reprogrammed to match the vehicle's specifications. This often requires specialized equipment and software

  8. 8. Inspect for Software Updates

    Check if there are any software updates available for the ECM or immobilizer system. Sometimes, manufacturers release updates to fix known issues

  9. 9. Consult Technical Service Bulletins (TSBs)

    Look for any TSBs related to the P1621 code for your specific vehicle make and model. Manufacturers may have documented solutions for common issues

  10. 10. Professional Diagnosis

    If the issue persists after performing the above steps, it may be necessary to seek assistance from a professional mechanic or dealership with experience in diagnosing and repairing ECM and immobilizer issues. By following these steps, you can systematically troubleshoot the P1621 code and identify the underlying cause of the problem