Conditions for Running the DTC

WARNING: This page is about a different car, the 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

Before the ECM can report DTC P2251 failed, DTC P0030, P0031, and P0032 must run and pass.
The ignition 1 voltage is between 10.7-18.0 volts.
The HO2S heater control is enabled.
The HO2S heater is at operating temperature for more than 5 seconds.
The internal HO2S sensing element resistance is more than 570 ohms.
The calculated exhaust temperature is less than 900°C (1652°F).
The ECM detects the internal HO2S signal voltage is between 1.47-1.53 volts.
DTC P2251 runs continuously once the above conditions are met.