Oxygen Sensor (O2S) Monitor

Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of feedback system is O2S. The O2S is located in exhaust path. Once it reaches operating temperatures of 300-350°C (572-662°F), sensor generates a voltage that is inversely proportional to amount of oxygen in exhaust. The information obtained by sensor is used to calculate fuel injector pulse width. The PCM is programmed to maintain optimum air/fuel ratio. At this mixture ratio, catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NOx) from exhaust.

The O2S is also main sensing element for EGR (if equipped), Catalyst and Fuel Monitors. The O2S may fail in any or all of the following manners:

• Slow Response Rate
• Reduced Output Voltage
• Dynamic Shift
• Shorted Or Open Circuits

Response rate is time required for sensor to switch from lean to rich once it is exposed to a richer than optimum A/F mixture or vice versa. As sensor starts malfunctioning, it could take longer to detect changes in oxygen content of exhaust gas.

The output voltage of O2S ranges from 0-1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in A/F mixture (lean or rich), output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond threshold value.