General Information

Component Location





General Description

The linear O2 sensor 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 O2 sensor produces a current that corresponds to a specific air/fuel ratio. The PCM monitors this signal and determines whether the air/fuel mixture is rich or lean. The PCM constantly monitors the linear O2 sensor and increases or decreases the fuel injection duration using this signal. This is called closed-loop fuel control operation.

DTC Description

The ECM monitors front O2 sensor signal level during fuel cut-off operation. This signal monitoring will determine open circuit of Reference Ground Circuit. The PCM sets DTC P2251 if front O2 signal during fuel cut-off is higher than predetermined threshold. If the same error code is set in the next driving cycle, the ECM illuminates the MIL.

DTC Detecting Condition





Schematic Diagram





Signal Waveform

1. Voltage in HO2S

Test Condition : Ignition ON, Engine OFF / HO2S disconnected





2. Scan Tool Data




Fig 1) Normal HO2S (B1S1) value with engine warm-up and idling : Approx. 1.5 - 2.0V
>Signal decreases with rich air/fuel mixture and increases with lean air/fuel mixture
>Fuel cut-off mode : Approx 3.2V