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 temperature 300-350°C (572-662°F), sensor generates a voltage that is inversely proportional to amount of oxygen in exhaust. When there is a large amount of oxygen in exhaust caused by a lean condition, sensor produces a low voltage, below 450 mV. When oxygen content is lower, caused by a rich condition, sensor produces a higher voltage, above 450 mV.

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 (Big Slope)
• Reduced Output Voltage (Half Cycle)
• Heater Performance

Slow Response Rate (Big Slope) -  Response rate is time required for sensor to switch from lean to rich signal output once it is exposed to a richer than optimum A/F mixture or vice versa. As PCM adjusts air/fuel ratio, sensor must be able to rapidly detect change. As sensor ages, it could take longer to detect changes in oxygen content of exhaust gas. The rate of change that an oxygen sensor experiences is called "Big Slope". The PCM checks oxygen sensor voltage in increments of a few milliseconds.

Reduced Output Voltage (Half Cycle) -  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. Each time voltage signal surpasses threshold, a counter is incremented by one. This is called Half Cycle Counter.

Heater Performance -  The heater is tested by a separate monitor. See Oxygen Sensor Heater Monitor  .