Slow Upstream O2 Sensor During Catalyst Monitor

Name of Code:
Slow Upstream O2 Sensor During Catalyst Monitor

When Monitored:
With the engine running, coolant greater than 17O°F, open throttle, steady to slightly increasing vehicle speed between 18 MPH and 55 MPH, with a light load on the engine, for a period no less than five minutes.

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 30O° to 350°C (572° to 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
- Weak upstream O2 sensor with deteriorated catalyst