LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Cadillac >> 1999 >> Catera >> Repair and Diagnosis >> Body & Frame >> Body, Cab Control Systems >> Body Control Modules >> System Tests >> Scan Tool Does Not Communicate With Class 2 Serial Data Line
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Scan Tool Does Not Communicate With Class 2 Serial Data Line

  1. Connect scan tool to DLC. Attempt to power up scan tool. If scan tool powers up, go to step  9 . If scan tool does not power up, go to next step.
  2. Disconnect scan tool from DLC. Check for poor connections between scan tool and DLC terminals No. 4 and 16. If poor connections are found, go to step  5 . If connections are okay, go to next step.
  3. Using DVOM, measure voltage between ground DLC terminal No. 16. If battery voltage is indicated, go to next step. If battery voltage is not indicated, go to step  7 .
  4. Using DVOM, measure resistance between ground and DLC terminal No. 4. If reading is less than 5 ohms, go to step  6 . If reading is greater than 5 ohms, go to step  8 .
  5. Repair connections as necessary. After repairs, go to step  1 .
  6. See scan tool manufactures instruction for proper operation of scan tool.
  7. Repair open in Red wire between HZD fuse (20-amp) located in fuse block and DLC terminal No. 16. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to step  1 .
  8. Repair open in Black wire between ground and DLC terminal No. 4. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to step  1 .
  9. Turn ignition on. Attempt to communicate with each module along the class 2 serial data line. If any modules communicate with scan tool, go to next step. If no modules communicate with scan tool, go to step  13 .
  10. If DTC U1128 is set in communicating modules, go to next step. If DTC U1128 is not set in communicating modules, go to step  12 .
  11. Go to DTC U1128: LOSS OF SERIAL COMMUNICATIONS - IRC  under DIAGNOSTIC TESTS.
  12. Go to BCM DIAGNOSTIC SYSTEM CHECK  .
  13. Turn ignition off. Check for poor connections between scan tool and DLC terminal No. 5. If connections are okay, go to next step. If poor connections are found, go to step  5 .
  14. Using DVOM, measure resistance between ground and DLC terminal No. 5. If reading is less than 5 ohms, go to next step. If reading is greater than 5 ohms, go to step  16 .
  15. Check for poor connections between scan tool and DLC terminal No. 2. If connections are okay, go to step  17 . If poor connections are found, go to step  5 .
  16. Repair open in Black wire between ground and DLC terminal No. 5. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to step  9 .
  17. Using DVOM, measure resistance between DLC terminals No. 2 and 4. If infinite (OL) resistance is indicated, go to next step. If any resistance is indicated, go to step  19 .
  18. Using DVOM, measure voltage between DLC terminals No. 2 and 4. Turn ignition on. Note voltage reading again. If both voltage readings are less than one volt, go to step  20 . If both voltage readings are not less than one volt, go to step  21 .
  19. Repair short to ground in Brown/White serial data wire. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to step  22 .
  20. Check power and ground circuits of non-communicating module. Repair as necessary. If power and ground circuits are okay, replace appropriate module.
  21. Turn ignition off. Repair short to voltage in Brown/White serial data wire. See appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS. After repairs, go to next step.
  22. Reconnect all disconnected components and connectors. Connect scan tool to DLC. Turn ignition on. Clear DTCs and recheck for DTCs. If DTCs clear, system is okay.
scientia non olet · About LEMON Manuals