The diagnostic codes sent by the Cadillac CTS computer illuminate the malfunction indicator lamp (MIL) for emissions related faults. The code is also stored in the power train control module (PCM) memory. You need a scan tool to read the codes, then the problem sensor must be replaced before you can clear the codes. If you try to clear the codes before you repair the problem, the light will come back on, because the PCM will identify the problem as still existing, and will send another code.
Plug the scan tool into the data link port, located under the drivers side dash, just to the left of the steering wheel.
Turn the key to the "On" position. Press the "Read" button. Write down the codes and match them to the code sheet to learn their meaning. Unplug the scan tool and turn the key off. Repair the problem as directed by the definition of the code on the code sheet.
Plug the scan tool into the data link port. Turn the key to the "On" position. Press the "Erase" button, and the scanner will delete the codes.
Test drive the vehicle. If the MIL light returns, repeat the procedure.
