Conditions for Setting the DTC

WARNING: This page is about a different car, the 2009 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The ECM detects that the an HO2S 2 is operating too lean while the ECM is commanding a rich air/fuel mixture for greater than 4 seconds.
OR
The ECM detects that an HO2S 1 is operating too rich while the corresponding HO2S 2 is operating too lean and the ECM detects that the fuel trim is at minimum control. The condition exists for greater than 4 seconds.