The oxygen sensor 1 (02S-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 powertrain control module (PCM) sends a bias voltage (391-491 mV) on the signal line which is pulled up through high resistance. When the O2S-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 O2S-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 mV and 999 mV. DTC P0131 sets when the O2S-1 signal voltage is too low for a certain length of time.
DTC Parameters
DTC P0131 will set if O2S-1 voltage is below 291 mV for 90 seconds when the following condition have been met:
OR
O2S-1 voltage is less than 291 mV in power enrichment mode for 9 seconds when the following condition have been met:
| • | The commanded air/fuel ratio is between 14.5:1-14.8:1. |
| • | The loop status is closed. |
| • | The transaxle is in gear (auto). |
| • | No cam (camshaft), crankshaft position (CKP), engine coolant temperature (ECT), exhaust gas recirculation (EGR), evaporative emission (EVAP) purge solenoid, fuel trim, intake air temperature (IAT), manifold absolute pressure (MAP), misfire or throttle position (TP) sensor DTCs have been set. |
DTC P0131 diagnostic runs continuously once the above conditions have been met.
DTC P0131 is a type B DTC.
Diagnostic Aids
To locate an intermittent problem, use a scan tool to monitor O2S-1 voltage with ignition ON, engine OFF. Wiggling wires while watching for a change from bias voltage (391-491 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 O2S-1 signal will be around 100 mV. The short term fuel trim value will be in the 160s as the system is near or at maximum correction.
Review Failure Record information to check if the vehicle was running lean in power enrichment -- the TP angle is greater than 75 percent, H02S-2 voltage below 300 mV, at time the DTC set. If this was the case, check for a possible lean condition.
