The heated oxygen (HO2S) bank 1 sensor 1 is threaded into the cylinder 1, 3, 5 exhaust manifold (cylinders 1, 3, 5 are closest to front of dash). The sensor monitors the oxygen content in the exhaust gas before the pre-converter. The ECM supplies a variable output pump current to circuit 1667 in order to maintain a constant voltage level at the input pump current circuit 1665. As output pump current passes through the oxygen sensor when the sensor is operating above 300°C (572°F), the sensor will vary resistance according to changes in exhaust oxygen content. If oxygen content increases (lean air/fuel ratio) the resistance of the sensor increases which requires more current to maintain a constant voltage level. The ECM calculates a lambda value (actual air/fuel ratio divided by 14.6 to 1) based on the input pump current voltage level on circuit 1665 and the signal voltage level on circuit 412. DTC P0131 will set when the HO2S bank 1 sensor 1 lambda value is too far out of range from the ECM modeled lambda value (comparison factor) for current operating conditions indicating the sensor is responding slowly.

DTC P0131 will set when the ECM modeled lambda comparison factor falls below 0.6 when:

DTC P0131 diagnostic runs continuously once the above conditions have been met.

P0131 is a (type B) DTC.

Bank 1 will default to open loop if this DTC is active.