DTC P2251-00: O2 Sensor Reference Ground Circuit/Open (Bank 1 Sensor 1): General Description

The linear HO2S is mounted on the front side of the catalytic converter (warm-up catalytic converter) or in the front exhaust pipe. It detects a wide range of air/fuel ratios in the exhaust gas from the rich to lean regions. This linear HO2S produces a current that corresponds to a specific air/fuel ratio. The ECM monitors this signal and determines whether the air/fuel mixture is rich or lean. The ECM constantly monitors the linear HO2S and increases or decreases the fuel injection duration using this signal. This is called closed-loop fuel control operation.