2004 Cadillac DTS code

You have a lean code for just one bank, or both? A lean code on just one bank could be O2 sensor, dirty injector, intake leak, or other vacuum leak that would only affect one side. If you have a lean code for both banks, you could have a skewed or dirty mass airflow sensor, leaking plenum, high alcohol contented fuel, an open vacuum source on the intake. Most typically, if it is both banks, the plenum boot between the intake and throttle has a split in it. This is usually on the bottom side, and can be somewhat intermittent. It can often be accompanied by a higher than normal idle speed.