For 1990-2009 cars

### Circuit Description

The oxygen sensor 1 (O2S-1) is 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 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 02S-1 signal used to adjust the 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 1065 mV. DTC P0131 sets when the O2S-1 signal voltage is too low for a certain length of time.

### Conditions for Setting the DTC

DTC P0131 will set if O2S-1 voltage is below 52 mV for 2 minutes and 5 seconds when the following conditions have been met for 14 seconds:

• | ECT is greater than 75°C (167°F)

• | Calculated airflow is greater than 3 g/s

• | TP angle is between 8 and 50%

• | No CKP, ECT, EVAP, fuel injector, fuel tank pressure, fuel trim, IAT, idle speed, MAP, misfire, PCM internal fault, or TP sensor DTCs have been set.

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

P0131 is a (type B) DTC

### Diagnostic Aids

To locate an intendment problem, use the scan tool to monitor the O2S-1 voltage with the ignition On, engine Off. Wiggling the wires while watching for a change from the 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 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.

### DTC P0131