Conditions for Setting the DTC

WARNING: This page is about a different car, the 2007 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.

Rich Test

The PCM detects that the HO2S 2 parameter is more than 1,000 mV.
DTC P0138 sets within 100 seconds during the rich test when the above condition is met.

Decel Fuel Cutoff Test

The PCM detects that the HO2S 2 parameter is more than 900 mV.
DTC P0138 sets within 40 seconds during the decel fuel cutoff test when the above condition is met.