Oxygen Sensor (O2S) Monitor

DESCRIPTION  Effective control of exhaust emissions is achieved by an oxygen sensor feedback system. The most important element of the feedback system are the oxygen (O2) sensors. The O2 sensors are located in the exhaust path. Once they reach operating temperature 300° to 350°C (572° to 662°F), the sensors generate a voltage that is inversely proportional to the amount of oxygen in the exhaust. When there is a large amount of oxygen in the exhaust caused by a lean condition, misfire or exhaust leak, the sensors produce a low voltage, below 450 mV. When the oxygen content is lower, caused by a rich condition, the sensors produce a higher voltage, above 450 mV.

The exhaust oxygen levels detected by the sensors is used to calculate the fuel injector pulse width. The PCM is programmed to maintain the optimum air/fuel ratio. At this mixture ratio, the catalyst is most effective at oxidizing (burning off) hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NOx) gasses from the exhaust.

The O2 sensors are also the main sensors that the PCM uses to monitor the EVAP Purge System, and Catalyst and Fuel Monitors. 

The O2S may fail in any or all of the following manners:

• Slow response rate
• Reduced output voltage
• Heater Performance
• Shorted or open circuits

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

Reduced Output Voltage  The output voltage of the O2S ranges from 0 to 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 the air/fuel mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value. Many times, the condition is only temporary and the sensor will recover.

OPERATION  As the Oxygen Sensor signal switches, the PCM monitors the half cycle and big slope signals from the oxygen sensor. If during the test neither counter reaches a predetermined value, a malfunction is entered and a Freeze Frame is stored. Only one counter reaching its predetermined value is needed for the monitor to pass.

The Oxygen Sensor Signal Monitor is a 2 trip monitor that is tested only once per trip. When the Oxygen Sensor fails the test in two consecutive trips, the MIL is illuminated and a DTC is set. The MIL is extinguished when the Oxygen Sensor monitor passes in three consecutive trips. The DTC is erased from memory after 40 consecutive warm-up cycles without test failure.