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DTC U1000 & U1255: Class 2 Communication Malfunction: Testing

WARNING: This page is about a different car, the 2001 GMC Safari and 2001 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
  1. Check for other "U" prefix DTCs. If DTC U1001-U1199 history code does not exist, go to next step. If DTC U1001-U1199 history code exists, go to DTC U1001-U1254: LOSS OF XXX COMMUNICATIONS  .
  2. Turn ignition switch to RUN position. Using scan tool, attempt communication with all modules along serial data line. If scan tool does not communicate with one or more modules, go to next step. If scan tool communicates with all modules, go to step  13 .
  3. Check for open or short to ground on power supply circuit of module(s) that is not communicating. See POWER DISTRIBUTION article in WIRING DIAGRAMS. If no problem is found, go to next step. If problem is found, repair as necessary. After repair, go to step  9 .
  4. Turn ignition switch to OFF position. Check for open on ground circuit of module that is not communicating. See GROUND DISTRIBUTION article in WIRING DIAGRAMS. If no problem is found, go to next step. If problem is found, repair as necessary. After repair, go to step  9 .
  5. Disconnect Natural 12-pin splice pack SP261 connector, located below instrument panel fuse block, taped to body harness. See Figure . Check for poor contact or damage. If no problem is found, go to next step. If problem is found, repair as necessary. After repair, go to step  9 .
  6. Turn ignition switch to OFF position. Check for open on data circuit of module that is not communicating. See DATA LINK CONNECTORS article in WIRING DIAGRAMS. If no problem is found, go to next step. If problem is found, repair as necessary. After repair, go to step  9 .
  7. Check for poor connection at power, ground and module communication circuits at affected module connector. See POWER DISTRIBUTION, GROUND DISTRIBUTION and DATA LINK CONNECTORS articles in WIRING DIAGRAMS for connector and terminal identification. If no problem is found, go to next step. If problem is found, repair as necessary. After repair, go to step  9 .
  8. Replace non-communicating module. See appropriate article for removal and installation procedure and programming, if necessary. Go to step  9 .
  9. Connect scan tool. Turn ignition switch to RUN position. Check for DTCs stored in module that is not communicating. If any DTCs exist that are not "U" prefix DTCs, go to appropriate article for diagnosis and repair. If only "U" prefix DTCs exist, go to next step.
  10. Using scan tool, clear DTCs. Go to next step.
  11. Check for DTCs stored in module that had DTC U1000 or U1255. If any DTCs exist that are not "U" prefix DTCs, go to appropriate article for diagnosis and repair. If only "U" prefix DTCs exist, go to next step.
  12. Using scan tool, clear DTCs. Test is complete.
  13. Check for DTCs stored in module that had DTC U1000 or U1255. If any DTCs exist, go to appropriate article for diagnosis and repair. If no other DTCs exist, go to next step.
  14. Connect scan tool. Turn ignition switch to RUN position. Clear DTCs. Turn ignition switch to OFF position for 5 seconds. Turn ignition switch to RUN position. Recheck for DTCs. If DTC U1000 or U1255 is current, go to next step. If DTC U1000 and U1255 is not current see DIAGNOSTIC AIDS  .
  15. Replace module indicated with DTC U1000 or U1255. Turn ignition switch to OFF position. Reinstall all components. Clear DTCs. Recheck for DTCs. If all DTCs are cleared, system is operating properly.
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