LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Cadillac >> 2019 >> CTS Base, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Data Communications - Diagnostic Information And Procedures >> Diagnostic Information and Procedures >> DTC U18A2 >> Circuit/System Testing
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Circuit/System Testing

NOTE:

Use the schematics and connector end views to identify the device's ground, B+, ignition, accessory wake up serial data, fuel pump primary relay control, fuel pump controller data out signal, and serial data circuit terminals.

  1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the K111 Fuel Pump Driver Control Module. It may take up to 10 min for all vehicle systems to power down.
    2. NOTE:

    If the control module's chassis ground is shared with any tail lamp assembly, disconnect the harness connector at that tail lamp assembly to remove the alternative path to ground before proceeding with this step.

  2. Test for less than 10 Ω between each ground circuit terminal and ground.
    • If 10 Ω or greater 
    1. Ignition/Vehicle OFF.
    2. Test for less than 2 Ω in the ground circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, repair the open/high resistance in the ground connection.
    • Go to next step: If less than 10 Ω 
  3. Ignition ON/Vehicle In Service Mode.
  4. If equipped, verify a test lamp illuminates between each B+ circuit terminal and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for less than 2 Ω in the B+ circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse.
    • If the test lamp does not illuminate and the circuit fuse is open 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for infinite resistance between the B+ circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, replace the K111 Fuel Pump Driver Control Module.
    • Go to next step: If the test lamp illuminates 
  5. If equipped, verify a test lamp illuminates between each ignition circuit terminal and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for less than 2 Ω in the ignition circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, verify the fuse is OK and there is voltage at the fuse.
    • If the test lamp does not illuminate and the circuit fuse is open 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for infinite resistance between the ignition circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, replace the K111 Fuel Pump Driver Control Module.
    • Go to next step: If the test lamp illuminates 
  6. If equipped, verify a test lamp illuminates between the accessory wakeup serial data circuit terminal (circuit 5985) and ground.
    • If the test lamp does not illuminate 
    1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K9 Body Control Module.
    2. Test for less than 2 Ω in the accessory wakeup serial data circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the K9 Body Control Module.
    • Go to next step: If the test lamp illuminates 
  7. If equipped, verify a test lamp illuminates between the powertrain sensor bus enable circuit terminal (circuit 4320) and ground.
    • If the test lamp does not illuminate 
    1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K20 Engine Control Module and all control modules connected to the powertrain sensor bus enable circuit.
    2. Test for infinite resistance between the powertrain sensor bus enable circuit terminal and ground.
      • If less than infinite resistance, repair the short to ground in the circuit.
      • Go to next step: If infinite resistance
    3. Test for less than 2 Ω in the powertrain sensor bus enable circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the K20 Engine Control Module.
    • Go to next step: If the test lamp illuminates 
  8. Verify if the K111 Fuel Pump Driver Control Module is equipped with the fuel pump primary relay control circuit (circuit 465).
    • If equipped 
    1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K20 Engine Control Module.
    2. Ignition ON/Vehicle In Service Mode.
    3. Test for less than 1 V between the fuel pump primary relay control circuit terminal and ground.
      • If 1 V or greater, repair the short to voltage in the circuit.
      • Go to next step: If less than 1 V
    4. Ignition/Vehicle OFF.
    5. Test for infinite resistance between the fuel pump primary relay control circuit terminal and ground.
      • If less than infinite resistance, repair the short to ground in the circuit.
      • Go to next step: If infinite resistance
    6. Test for less than 2 Ω in the fuel pump primary relay control circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the K20 Engine Control Module.
    • Go to next step: If NOT equipped 
  9. Ignition ON/Vehicle In Service Mode.
  10. Test for less than 4.5 V between each powertrain high speed GMLAN serial data circuit terminal and ground.
    • Circuit 7493
    • Circuit 7494
    • If 4.5 V or greater 

      Refer to DTC U0074  to test for a short to voltage.

    • Go to next step: If less than 4.5 V 
  11. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
  12. Test for greater than 100 Ω between each powertrain high speed GMLAN serial data circuit terminal and ground.
    • Circuit 7493
    • Circuit 7494
    • If 100 Ω or less 

      Refer to DTC U0074  to test for a short to ground.

    • Go to next step: If greater than 100 Ω 
  13. Disconnect the harness connectors at the device setting the DTC.
  14. Test for less than 2 Ω in each serial data circuits end to end between the K111 Fuel Pump Driver Control Module and the device setting the DTC.
    • Circuit 7493
    • Circuit 7494
    • If 2 Ω or greater 

      Repair the open/high resistance in the serial data circuit.

    • Go to next step: If less than 2 Ω 
  15. Replace the K111 Fuel Pump Driver Control Module.
  16. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
  17. Ignition ON/Vehicle In Service Mode.
  18. Verify DTC U18A2 does not set again.
    • If DTC U18A2 sets 

      Replace the K20 Engine Control Module.

    • Go to next step: If DTC U18A2 does not set 
  19. All OK.
scientia non olet · About LEMON Manuals