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HOMESERVICE MANUALSPONTIAC1997GRAND PRIX GTP, 4D SEDANREPAIR AND DIAGNOSIS (SINGLE PAGE)BRAKESANTI-LOCK BRAKE SYSTEM WITH TRACTION CONTROL SYSTEM - ABS-VIDIAGNOSTIC TROUBLE CODES (DTCS)DTC U1255: CLASS 2 COMMUNICATIONS MALFUNCTION
1997 Pontiac Grand Prix GTP, 4D Sedan

DTC U1255: Class 2 Communications Malfunction

1997 Pontiac Grand Prix GTP, 4D SedanSECTION DTC U1255: Class 2 Communications Malfunction
  1. Perform diagnostic system check. See DIAGNOSTIC SYSTEM CHECK . After performing diagnostic system check, go to next step.
  2. Using scan tool, attempt to establish communication with same system on a different vehicle. If communication can be established, go to next step. If communication cannot be established, refer to scan tool operation manual for scan tool diagnosis.
  3. Turn ignition off. Check Data Link Connector (DLC) for poor terminal contact, corrosion or damage. If connector is faulty, go to step 14). If connector is okay, go to next step.
  4. Turn ignition on, with engine off. Install scan tool. Using scan tool, attempt to establish communication with other modules connected to serial data line. If communication can be established, go to step 12). If communication cannot be established, go to next step.
  5. Turn ignition off. Disconnect instrument cluster, heater-A/C control, Body Control Module (BCM), and Powertrain Control Module (PCM). Disconnect 24-pin EBCM/EBTCM connector C1 and 8-pin EBCM/EBTCM connector C2. Using DVOM, measure resistance between terminals No. 2 (Purple wire) and 4 (Black wire) of DLC. See WIRING DIAGRAMS . If resistance is infinite, go to next step. If resistance is not infinite, go to step 15).
  6. Turn ignition on, with engine off. Using DVOM, measure voltage between terminals No. 2 (Purple wire) and 4 (Black wire) of DLC. See WIRING DIAGRAMS . If voltage is not present, go to next step. If voltage is present, go to step 16).
  7. Turn ignition off. Reconnect instrument cluster. Turn ignition on, with engine off. Using scan tool, attempt to establish communication with instrument cluster. If communication can be established, go to next step. If communication cannot be established, go to INSTRUMENT PANEL article in ACCESSORIES & EQUIPMENT section for diagnosis.
  8. Turn ignition off. Reconnect heater-A/C control. Turn ignition on, with engine off. Using scan tool, attempt to establish communication with heater-A/C control. If communication can be established, go to next step. If communication cannot be established, see A/C-HEATER SYSTEM TROUBLE SHOOTING - MANUAL article in AIR CONDITIONING & HEAT section for diagnosis.
  9. Turn ignition off. Reconnect BCM. Turn ignition on, with engine off. Using scan tool, attempt to establish communication with BCM. If communication can be established, go to next step. If communication cannot be established, go to BODY CONTROL MODULE article in ACCESSORIES & EQUIPMENT section for diagnosis.
  10. Turn ignition off. Reconnect PCM. Turn ignition on, with engine off. Using scan tool, attempt to establish communication with PCM. If communication can be established, go to next step. If communication cannot be established, go to step 19).
  11. Turn ignition off. Reconnect EBCM/EBTCM connectors C1 and C2. Turn ignition on, with engine off. Using scan tool, attempt to establish communication with EBCM/EBTCM. If communication can be established, go to step 20). If communication cannot be established, go to step 18).
  12. Turn ignition off. Disconnect 24-pin EBCM/EBTCM connector C1. Check EBCM/EBTCM connector C1 for poor terminal contact, corrosion or damage. If connector is faulty, go to step 14). If connector is okay, go to next step.
  13. Using DVOM, measure resistance between terminal No. 2 (Purple wire) of DLC and terminal A1 (Light Blue wire) of EBCM/EBTCM connector C1. See WIRING DIAGRAMS . If resistance is 0-2 ohms, go to step 18). If resistance is not 0-2 ohms, go to step 17).
  14. Replace all faulty terminals or connectors. After repairs, perform DIAGNOSTIC SYSTEM CHECK .
  15. Repair short to ground in DLC Purple wire or Light Blue wire circuit. After repairs, perform DIAGNOSTIC SYSTEM CHECK .
  16. Repair short to voltage in DLC Purple wire or Light Blue wire circuit. After repairs, perform DIAGNOSTIC SYSTEM CHECK .
  17. Repair open in Purple wire or Light Blue wire between DLC and EBCM/EBTCM. After repairs, perform DIAGNOSTIC SYSTEM CHECK .
  18. Replace EBCM/EBTCM. After repairs, perform DIAGNOSTIC SYSTEM CHECK .
  19. Perform PCM ON-BOARD DIAGNOSTIC SYSTEM CHECK. See TESTS W/CODES - 3.1L article.
  20. Malfunction is not currently present. When DTC U1255 is current (malfunction in Purple wire, Light Blue wire or Dark Green wire circuit), no communication can be established with scan tool. When malfunction is present, DTC U1255 will be stored in system memory. Before performing diagnostic testing for DTC U1255, perform PCM on-board diagnostic system check to eliminate possible faulty PCM. See TESTS W/CODES - 3.1L article. An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside insulation. Frequency of malfunction can be determined by using enhanced diagnostic function of scan tool. Check for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wiring connections, or damaged wiring harness.
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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.