Upstream O2 Sensor Slow Response

Name of Code:
Upstream O2 Sensor Slow Response

When Monitored:
With the engine running for more than 3 minutes, engine temperature greater than 120°F, after reaching a vehicle speed of 20 mph for 120 seconds, at idle with the transmission in drive (auto) or in neutral (manual).

Set Condition:
The upstream oxygen sensor signal voltage is switching from below 0.39 volt to above 0.6 volt and back fewer times than required.

Theory of Operation:
Effective control of exhaust emissions is achieved by an oxygen feedback system, of which the most important element is the upstream oxygen sensor (O2S), which is located in the exhaust path. Once the sensor reaches its normal operating temperature of 300°C-350°C (572°F - 662°F), it generates a voltage inversely proportional to the amount of oxygen in the exhaust. This voltage is used to calculate the fuel injector pulse width, which maintains the optimum 14.7 to 1 air/fuel (A/F) ratio that is required for the catalytic converter to best control exhaust emissions. 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 than optimum 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
- Dirt/moisture/grease inside connector
- Fuel contamination
- Engine mechanical