Catalyst Monitor: Operation

OPERATION 

The State of Change (SOC)  catalyst monitor uses the signals from both the Upstream and Downstream Oxygen (O2) Sensors to detect aging of the catalyst. The Upstream O2 Sensor slow response monitor runs simultaneously to determine if the sensor signal is plausible. A pass/fail determination is made on the O2 Sensor slow response diagnostic before a catalyst determination is made to prevent a false fail of the catalyst monitor. These monitors are intended to determine when the O2 Sensor and catalyst systems have deteriorated to the point that tailpipe emissions exceed the appropriate malfunction limit.

Inside the Catalytic Converter, the gases flow through a dense ceramic honeycomb structure coated with the catalysts. The honeycomb structure allows the gases to touch a larger area of catalyst at once, so they are converted more quickly and efficiently. The three way catalysts work to reduce tailpipe emissions.

• The catalyst works by combining oxygen with Carbon Monoxide (CO) and unburned hydrocarbons (HC) to produce Carbon Dioxide (CO2) and water.
• The three way catalyst also reduces Oxides of Nitrogen (NOx) using a chemical process to produce nitrogen and oxygen gases.

The Catalytic Converter operates most efficiently when the engine is operated within a narrow band of air/fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen). The Upstream O2 Sensor feedback is used to maintain this optimal air/fuel ratio. The Downstream O2 Sensor feedback is used to monitor the efficiency of the Catalytic Converter.

Switching (4-wire) O2 Sensor Operation 

• O2 Sensor Heater Operation  For a typical Switching (4-wire) Oxygen (O2) Sensor output signal to function the sense element must be heated to operating temperature. A resistive heater element is incorporated within the sensor to allow for rapid heating of the sensor to reach operating temperature faster and closed loop fuel control. The O2 Sensor Heater resistance is directly proportional to the heater temperature, meaning that as heater temperature increases, the heater resistance increases. The Powertrain Control Module (PCM) makes a calculation to determine the heater resistance and temperature. The heater resistance is calculate by measuring the Battery voltage and current draw on the heater control circuit, then the heater temperature is determined using the resistance versus temperature characteristic. The heater temperature is monitored continuously and the heating rate is adjusted using Pulse Width Modulation (PWM) to prevent damaging the heating element. If the PCM detects a fault in the O2 Sensor heater circuitry the internal PCM driver is disabled during the current ignition cycle.
• O2 Sensor Output Operation  Normal range of the O2 Sensor output is a 0 to 1.0 volt Analog to Digital (A/D) signal when the sensor is in normal operating temperature range. The output voltage is generated by comparing the Oxygen content in the atmosphere, collected in a reference chamber to the Oxygen content in the exhaust stream collected in a comparison chamber. When the oxygen content is high (caused by a lean air/fuel mixture) the sensor produces a low voltage. When the oxygen content is low (caused by a rich air/fuel mixture) it produces a higher voltage. In some instances a negative offset output of up to -1.0 volts may be introduced if the sensors reference chamber is contaminated. To allow for the negative voltage to be read, each O2 Sensor Return circuit has a 2.5 volt bias added to shift the signal voltage to between 2.5 volts and 3.5 volts.

Wide-band O2 Sensor Operation 

• O2 Sensor Heater Operation  For the wide-band O2 Sensor to deliver accurate readings, the sensing elements must be heated. A Positive Temperature Coefficient (PTC) element inside the O2 Sensor heats up as current passes through it. This allows the system to enter Closed Loop quickly. The Powertrain Control Module (PCM) turns on this circuit based on Engine Coolant Temperature (ECT) and engine loads. The PCM monitors the O2 Sensors heater resistance for circuit fault detection.
• O2 Sensor Output Operation  The wide-band O2 Sensor operates differently than traditional O2 Sensors. 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 Reference 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 Reference voltage and O2 Sensor Signal voltage will be a fixed voltage value. The O2 Sensor Current Pump voltage will switch from between 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 should maintain a steady 0.45 volts when taking a voltage measurement between the O2 Sensor Signal circuit and the O2 Sensor Reference 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 pumping element 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 pumping element voltage will move toward -0.45 volts pumping oxygen into the measurement chamber.