Description And Operation
DESCRIPTION
This vehicle is equipped with two Oxygen (O2) sensors:
Upstream O2 sensor (6 wire)
The upstream O2 sensor also designated (1/1) is located before the catalytic converter and has a 6 wire connector. The upstream O2 sensor is a fast-acting wide band O2 sensor that measures oxygen content in the exhaust gas. The sensing element uses zirconium dioxide (ZrO2), or zirconium, in a ceramic substrate. The sensing element creates a voltage in response to oxygen levels. The upstream O2 sensor also contains an oxygen pumping function that provides the wide-band sensing capabilities. The pumping feature is critical for proper oxygen measurement. The sensing element is bonded to an alumina ceramic layer that contains a heating element. The quick-acting heater allows the sensor to reach operating temperature in less than 8 seconds. The operating temperature ranges from 600°C to 830°C (1112°F to 1526°F). The sensing element can withstand temperatures of up to 950°C (1742°F). The connector is completely sealed and waterproof as there is no need for a reference sample of atmospheric oxygen.
Downstream O2 sensor (4 wire)
The downstream O2 sensor also designated (1/2) is located after the catalytic converter and has a 4 wire connector. The downstream O2 sensor is equipped with a galvanic battery that typically generates a voltage signal between 0.0 volts and 1.0 volts. The 4-wire O2 sensor also includes a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.
OPERATION
The voltage signals are proportional to the amount of unburned oxygen in the exhaust. When hot, at least 316°C (600°F), the zirconium dioxide element in the sensor's tip produces a voltage signal that varies according to the difference in oxygen content between exhaust and outside air. The higher the concentration of unburned oxygen in the exhaust, the lower the differential across the sensor tip and the lower the sensor's voltage output. The sensors output ranges from 0.1 volts (lean) to 0.9 volts (rich). A perfectly balanced (stoichiometric) fuel mixture of 14.7:1, gives a reading of around 0.5 volt
Upstream O2 sensor (6 wire)
The wide-band (6-wire) O2 sensor operates differently than the traditional 4-wire O2 sensor. The wide-band O2 sensor tip consists of two cells that provide different functions, a measurement chamber and a detection chamber with pumping capabilities. The oxygen pumping function is the ability to pump oxygen into or out of the measurement chamber depending on the level of oxygen in the measurement chamber. This function provides the wide-band sensing capabilities and is critical for proper oxygen measurement. The O2 sensor positive current control circuit provides a common bias supply to both the O2 sensor signal and the O2 sensor pump cell current circuits.
During normal operation, the O2 sensor positive current control voltage and O2 sensor signal voltage will be a fixed voltage value. The O2 sensor pump cell current voltage will switch from approximately 0.45 volts above and below the fixed O2 sensor return voltage, allowing current to flow in either direction through the pump. This correlates with the pumping of oxygen into and out of the measurement chamber. On a properly operating vehicle, this happens very quickly and the voltage reading will appear as a steady 0.45 volts when taking a voltage measurement between the O2 sensor signal circuit and the O2 sensor positive current control circuit of the O2 sensor with the engine running and the O2 sensor operating in closed loop.
When the exhaust stream has a lean air/fuel ratio (high oxygen content) the pump cell current voltage will move toward +0.45 volts pumping oxygen out of the measurement chamber. When the exhaust stream has a rich air/fuel ratio (relatively low oxygen content) the pump cell current voltage will move toward -0.45 volts pumping oxygen into the measurement chamber.
The Powertrain Control Module (PCM) is able to pinpoint failures to the specific O2 sensor circuits through a series of steps which include both "cold" and "hot" tests. To ensure that the correct malfunctioning circuit is identified, the diagnostics are performed in three separate test conditions, a cold test at startup, hot test during vehicle operation and cold test during PCM power down.
Downstream O2 sensor (4 wire)
The 4-wire O2 sensor typically generates a voltage signal between 0.0 volts and 1.0 volts. The 4-wire O2 sensor also includes a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. It also allows the system to remain in closed loop operation during periods of extended idle.
A bias voltage in the PCM shifts the signal voltage to fluctuate between 2.5 volts and 3.5 volts, depending upon the oxygen content of the exhaust gas. When a large amount of oxygen is present (caused by a lean air/fuel mixture) the sensor produces a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch.
In open loop operation the PCM ignores input from the O2 sensors. In closed loop operation the PCM monitors the O2 sensors input (along with other inputs) and adjusts the injector pulse width accordingly based on pre-programmed (fixed) values and inputs from other sensors. The PCM also compares upstream and downstream O2 sensor inputs to calculate the catalytic convertor oxygen storage capacity and converter efficiency.
The 4-wire downstream O2 sensor serves two functions. The first function is to measure the catalyst efficiency. If the catalytic converter is working properly, the oxygen content of the exhaust gases at the converter outlet fluctuate significantly less than the converter inlet. The PCM compares the switching rates of both downstream and upstream O2 sensors under specific operating conditions to determine if the catalyst is functioning properly. Anytime the upstream to downstream switching ratio exceeds a calibrated value, a catalyst efficiency Diagnostic Trouble Code (DTC) is set.
The second function of the downstream O2 sensor is downstream fuel control and is active only during downstream closed loop operation. This function trims the 4-wire upstream O2 sensor goal voltage within the range of upstream O2 sensor operation. The upstream O2 sensor goal voltage is modified to ensure long catalytic converter life by allowing the PCM to control the amount of oxygen that is supplied to the catalytic converter. The downstream O2 sensor is a good indicator of how much oxygen remains after all of the reactions take place in the converter.