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U1798SCP (J1850) Lack of Acknowledgment for External Access (Doors)

The OBD2 code U1798 SCP (J1850) indicates a lack of acknowledgment from the vehicle's external access system, specifically related to the doors, suggesting a communication issue between modules

Definition

The OBD2 code U1798 SCP (J1850) indicates a lack of acknowledgment from the vehicle's external access system, specifically related to the doors, suggesting a communication issue between modules

Common causes

  • Faulty communication between modules
  • Damaged wiring or connectors
  • Malfunctioning external access module
  • Software issues or bugs in the vehicle's control modules
  • Low battery voltage or power supply issues
  • Corroded or loose ground connections
  • Interference from aftermarket devices or modifications

Common misdiagnoses

  • Faulty door module
  • Wiring issues or shorts
  • Malfunctioning body control module (BCM)
  • Incorrectly programmed or updated software
  • Faulty key fob or remote entry system
  • Battery or power supply issues
  • Ground connection problems
  • Corroded connectors or terminals

Troubleshooting steps

  1. 1.Check for Other Codes

    Scan the vehicle for any additional trouble codes. Sometimes, related codes can provide more context for the issue

  2. 2.Inspect Wiring and Connectors

    Visual Inspection: Check the wiring harnesses and connectors associated with the door modules and the J1850 bus for any signs of damage, corrosion, or loose connections

  3. 3.Repair or Replace

    Repair any damaged wires or connectors as necessary

  4. 4.Test the Modules

    Module Functionality: Verify that the door modules are functioning correctly. This may involve checking for proper operation of door locks, windows, and other related features

  5. 5.Communication Check

    Use a scan tool to check if the modules are communicating properly. If a specific module is not responding, it may need to be replaced

  6. 6.Check the SCP Bus

    Bus Integrity: Use an oscilloscope or a multimeter to check the integrity of the SCP bus. Look for proper voltage levels and signal integrity

  7. 7.Termination Resistors

    Ensure that the termination resistors are present and functioning correctly at both ends of the bus

  8. 8.Inspect for Short Circuits

    Shorts to Ground or Power: Check for any short circuits in the wiring that could be affecting communication. This includes checking for shorts to ground or power in the wiring harness

  9. 9.Check for Interference

    Electrical Interference: Look for any aftermarket devices or modifications that may be causing electrical interference with the communication bus

  10. 10.Perform a Hard Reset

    Battery Disconnect: Disconnect the vehicle's battery for a few minutes to reset the modules. Reconnect and see if the code reappears

  11. 11.Update Software/Firmware

    Module Updates: Check if there are any software or firmware updates available for the vehicle's modules. Sometimes, manufacturers release updates to fix communication issues

  12. 12.Consult Technical Service Bulletins (TSBs)

    Manufacturer TSBs: Look for any TSBs related to the U1798 code or communication issues for your specific vehicle make and model. There may be known fixes or recalls

  13. 13.Professional Diagnosis

    If the issue persists after performing the above steps, consider seeking help from a professional mechanic or dealership with experience in diagnosing communication issues. By following these troubleshooting steps, you should be able to identify and resolve the underlying cause of the U1798 code