1999 Cadillac Seville SLSFACTORY OEM SERVICE PROCEDURE

DTC U1255: Class 2 Serial Data Line Initialization Error: Testing

  1. If data link communications diagnostic system check has been performed, go to next step. If data link communications diagnostic system check has not been performed, perform data link communications diagnostic system check. See DATA LINK COMMUNICATIONS DIAGNOSTIC SYSTEM CHECK  under SYSTEM TESTS.
  2. Determine which module(s) on class 2 data line caused DTC to set by checking Data Link Connector (DLC) circuit. Use scan tool to attempt and establish communication with suspect module(s). If scan tool communicates with affected module, go to next step. If scan tool does not communicate with affected module, go to appropriate diagnostic and testing procedure for particular module.
  3. Check and repair class 2 serial data line circuit No. 1807 (Purple wire) for intermittent conditions. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to next step.
  4. Turn ignition switch to OFF position. Reconnect components and connectors. Turn ignition switch to ON position. Using scan tool, clear DTCs. After clearing DTCs, perform DATA LINK COMMUNICATIONS DIAGNOSTIC SYSTEM CHECK  under SYSTEM TESTS.
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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.