Conditions for Setting the DTC

WARNING: This page is about a different car, the 2009 Pontiac Torrent and 2009 Chevrolet Equinox. However, it is still accessible from the selected car via links, so may be relevant.

The ECM detects that the HO2S 1 voltage is less than 60 mV while the corresponding HO2S 2 voltage is greater than 500 mV.
The ECM detects that the HO2S 1 voltage is less than 60 mV after a cold start with the engine coolant temperature (ECT) less than 40°C (104°F).