The heated oxygen sensor 1 (HO2S-1) is an electrical source that responds to oxygen content in the exhaust manifold. When the sensor reaches approximately 316°C (600°F), it produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The PCM sends a bias voltage (399-499 mV) on the signal line, which is pulled up through high resistance. When the HO2S-1 is cold, it produces no voltage and has extremely high internal resistance. The internal resistance of the sensor is much greater than the resistance of the bias pull-up resistor. However, when the sensor heats up, it produces voltage that overrides the bias voltage. This voltage is read by the PCM to determine a rich/lean HO2S-1 signal used to adjust injector pulse width. Under normal conditions, low sensor voltage means high oxygen content/lean air-fuel mixture and vice versa. Normal sensor readings will fluctuate between 10-1065 mV. DTC P0131 sets when the HO2S-1 signal voltage is too low for a certain length of time.

DTC P0131 will set if HO2S-1 voltage is below 52 mV for 2 minutes and 5 seconds when the following conditions have been met for 14 seconds:

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

P0131 is a type B DTC

To locate an intermittent problem, use the scan tool to monitor the HO2S-1 voltage with the ignition ON and the engine OFF. Wiggling the wires while watching for a change from bias voltage (399-499 mV) may locate the area where a short to ground in the wiring could lie.

If the system is commanding rich (engine running lean) the HO2S-1 signal will be around 100 mV. The short term fuel trim value will be in between 160-169 as the system is near or at maximum correction.