Downstream O2 Sensor Slow Response

NAME OF CODE:
Downstream O2S Response

WHEN MONITORED:
With the engine running for more than 3 minutes, engine temperature greater than 170°F, after reaching a vehicle speed of 24 MPH, at idle with the transmission in neutral.

SET CONDITION:
The downstream oxygen sensor signal voltage is switching between 1.0 volt and 0.5 volt inversely proportional to the upstream O2S but at a lower rate.

THEORY OF OPERATION:
Effective control of exhaust emissions is achieved by an oxygen feedback system. The downstream oxygen sensor (O2S), which is located in the exhaust path, monitors the exhaust emissions. Once the sensor reaches its normal operating temperature of 300-350° C (572-662 F), it generates a voltage inversely proportional to the amount of oxygen in the exhaust. This voltage is used to calculate the efficiency of the catalytic converter. Response rate is the time required for the sensor to detect changes in the exhaust gas oxygen content; it switches from lean to rich if it is exposed to a richer air/fuel (A/F) mixture, or vice versa. The O2S output voltage ranges from 0 to 1 volt. A good sensor easily generates an output voltage in this range as it is exposed to different oxygen concentrations. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the thresholds.

POSSIBLE CAUSES:
- Exhaust leak
- Fuel contamination
- Electrical wiring connectors
- Engine mechanical