UART Serial Data Line

If all components are not accessible with a scan tool, check for an open in circuit 800 between DLC terminal No. 9 and splice S277 or a short to battery or ground.
If a single component is not accessible with a scan tool, check for an open in circuit 800 between S277 and the component.
Visually inspect for obvious signs of mechanical and electrical damage. Inspect for blown fuses and damaged relays. Inspect for loose or corroded connections, damaged wiring harnesses and/or switches. Check for a broken or partially broken wire inside insulation, which could cause system malfunction but prove good in a continuity/voltage check with system disconnected. These circuits may be intermittent or resistive when loaded and should be checked by monitoring the voltage drop with the system under load.
Ensure any aftermarket electronic equipment is properly installed. If fault is found, repair as necessary.