U1798 – SCP (J1850) Lack of Acknowledgment for External Access (Doors)

The ODBII 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.

The severity of ODBII code U1798 is moderate.

Common Causes For: U1797 SCP (J1850) Lack of Acknowledgment for Door Locks

• 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 misdiagnosis for U1798 SCP (J1850) Lack of Acknowledgment for External Access (Doors)

• 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

Common Troubleshooting Steps

The OBD-II code U1798 indicates a communication issue related to the SCP (Serial Communication Protocol) on the J1850 bus, specifically indicating a lack of acknowledgment for external access, which can often be related to door modules or other components that communicate over this protocol. Here are some common troubleshooting steps to diagnose and resolve this issue:

• Check for Other Codes: Scan the vehicle for any additional trouble codes. Sometimes, related codes can provide more context for the issue.
• 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.
• **Repair or Replace: Repair any damaged wires or connectors as necessary.
• 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.
• **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.
• 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.
• **Termination Resistors: Ensure that the termination resistors are present and functioning correctly at both ends of the bus.
• 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.
• Check for Interference: **Electrical Interference: Look for any aftermarket devices or modifications that may be causing electrical interference with the communication bus.
• 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.
• 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.
• 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.
• 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.