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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 24 (BODY CONTROL MODULES)DIAGNOSTIC TESTSDTC U1000 OR U1255: CLASS 2 COMMUNICATION MALFUNCTIONTESTING
1999 GMC Forward Control 5.7 R, Automatic

DTC U1000 Or U1255: Class 2 Communication Malfunction: Testing

1999 GMC Forward Control 5.7 R, AutomaticSECTION Testing
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2002 GMC Savana, 2002 Chevrolet Chevy Express, 2001 GMC Savana Special, 2001 GMC Savana Camper Special, and 2001 GMC Savana. However, it is still accessible from the selected car via links, so may be relevant.
  1. If no history DTCs between DTC U1001 and U1199 exist, go to next step. If any history DTCs between DTC U1001 and U1199 exist, go to DTC U1001-U1254: LOSS OF XXX COMMUNICATION  .
  2. Turn ignition switch to RUN position. Using scan tool, attempt to communicate with each module on class 2 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 in power circuits, ignition inputs or ignition outputs for non-communicating module. See POWER DISTRIBUTION article in WIRING DIAGRAMS. If power circuits are okay, go to next step. If faulty circuit is found, repair as necessary. After repair, go to step  9 .
  4. Turn ignition switch to OFF position. Check for open in ground circuit(s) for non-communicating module. See GROUND DISTRIBUTION article in WIRING DIAGRAMS. If circuit is okay, go to next step. If faulty circuit is found, repair as necessary. After repair, go to step  9 .
  5. Disconnect splice connector S207, located in instrument panel harness, approximately 2 in from park brake warning switch breakout, toward data link connector (DLC) breakout, and splice S153, located in engine harness approximately 3 in from C100 breakout toward PCM breakout (except 6.5L), or in engine harness, approximately 10 in from A/C compressor clutch breakout toward C100 breakout (6.5L). Check for continuity in Purple wire between appropriate module and S207 and between S207 and S153. If circuit(s) are okay, go to next step. If circuit(s) are not okay, repair as necessary. After repair, go to step  9 .
  6. Check for open in class 2 serial data line (Purple wire) for non-communicating module(s). If circuit is okay, go to next step. If circuit is not okay, repair as necessary. After repair go to step  9 .
  7. Check for open or short to ground in battery voltage, ignition input, ignition output, grounds and class 2 serial data circuits for non-communicating module. If circuits are okay, go to next step. If circuits are not okay, repair circuit(s) as necessary. After repair, go to step  9 .
  8. Replace non-communicating module. After repair, go to step  11 .
  9. Connect scan tool to Data Link Connector (DLC). Turn ignition switch to RUN position. Select display DTCs function for non-communicating module. If scan tool does not display any DTCs that do not begin with a "U", go to next step. If scan tool displays any DTCs that do not begin with a "U", perform appropriate diagnostic system check. See appropriate article for malfunctioning system.
  10. Using scan tool, clear DTCs. Go to next step.
  11. Select display DTCs function for module that had DTC U1000 or U1255 set as current. If scan tool displays any DTCs that do not begin with a "U", perform appropriate diagnostic system check. See appropriate article for malfunctioning system. If scan tool does not display any DTCs that do not begin with a "U", go to next step.
  12. Using scan tool, clear DTCs. System is okay.
  13. If no other DTCs exist, go to next step. If any other DTCs exist, perform appropriate diagnostic system check. See appropriate article for malfunctioning system.
  14. Connect scan tool to Data Link Connector (DLC). Turn ignition switch to RUN position. Using scan tool, clear DTCs. Turn ignition switch to OFF position and wait at least 5 seconds. Turn ignition switch to RUN position. Select display DTCs function. If DTC U1000 or U1255 is set as current, go to next step. If DTC U1000 or U1255 is not set as current, go to DIAGNOSTIC AIDS  .
  15. Replace module that had DTC U1000 or U1255 set as current.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.