Oxygen Sensor: Description and Operation
DescriptionFour Heated Oxygen Sensors (HO2S) are mounted in the exhaust system where they monitor the oxygen content in the exhaust stream. There is an oxygen sensor mounted in each exhaust manifold and one on each end of the catalytic converter. The oxygen sensor located in the front exhaust manifold is the Bank 2 Sensor 1 (Front) sensor. The oxygen sensor located in the rear exhaust manifold is the Bank 1 Sensor 1 (Rear) sensor. The oxygen sensor located in the Y-pipe ahead of the catalytic converter is the Bank 1 Sensor 2 (Pre-converter) sensor. The oxygen sensor located in the catalytic converter outlet is the Bank 1 Sensor 3 (Post-converter) sensor.
Operation
An oxygen sensor acts like a battery because it creates its own signal voltage once it reaches operating temperature. This voltage is produced when the oxygen content in the exhaust stream is different than the oxygen content in the atmosphere. A lean condition (high oxygen content in exhaust) will produce a low voltage (near 0 volts) and a rich condition (low oxygen content in exhaust) will produce a high voltage (near one volt). The PCM provides a reference signal voltage (0.45 volt) and a ground to the sensor. The PCM reference voltage is necessary because the oxygen sensors do not provide their own voltage until they reach operating temperature.
Fuel Control Heated Oxygen Sensors (Bank 1, HO2S 1) (Bank 2, HO2S 1)
The main function of the fuel control heated oxygen sensor is to provide the control module with exhaust stream information in order to allow proper fueling and maintain emissions within the mandated levels. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.
The control module uses the signal voltage from the fuel control heated oxygen sensors in a closed loop in order to adjust the fuel injector pulse width. While in a closed loop, the Control Module can adjust fuel delivery in order to maintain an air to fuel ratio which allows the best combination of emission control and driveability.
If the oxygen sensor pigtail wiring, connector or terminal are damaged, replace the entire oxygen sensor assembly. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, the sensor must have a clean air reference provided to it. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference. Any attempt to repair the wires, connectors or terminals could degrade oxygen sensor performance.
Catalyst Monitor Heated Oxygen Sensors (Bank 1 HO2S 2 and Bank 1 Sensor 3)
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HO and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The Control Module has the ability to monitor this process using the Bank 1 HO2S 2 and the Bank 1 HO2S 3 heated oxygen sensors. The Bank 1 HO2S 2 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 2 signal will be far more active than that produced by the Bank 1 HO2S 3 sensor.
In addition to catalyst monitoring, the Bank 1 HO2S 3 heated oxygen sensor has a limited role in controlling fuel delivery. If the Bank 1 HO2S 3 signal indicates a high or low oxygen content for an extended period of time while in a closed loop, the Control Module adjusts the fuel delivery slightly in order to compensate.
Catalyst Monitor Diagnostic Operation
The ODBII catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the Three-Way Catalyst (TWC). Voltage variations between the sensors allow the control module to determine the catalyst emission performance.
As a catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on an correlation between conversion efficiency and oxygen storage capacity.
A good catalyst (e.g. 95% hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S). A degraded catalyst (65% hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.
The post-catalyst HO2S 2 is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC and both the Pre and Post Converter HO2S must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the Post-Catalyst HO2S 3 Outputs graphic.
The catalyst monitor diagnostic is sensitive to the following conditions:
^ Exhaust leaks
^ HO2S Contamination
^ Alternate fuels
Exhaust system leaks may cause the following results:
^ Prevent a degraded catalyst from failing the diagnostic
^ Cause a false failure for a normally functioning catalyst
^ Prevent the diagnostic from running Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants prevents the TWC diagnostic from functioning properly.
Three-Way Catalyst Oxygen Storage Capacity
Notice: Numbers in text correspond to numbered components in image.
The control module must monitor the Three-Way catalyst system (TWC) for efficiency. In order to accomplish this, the control module monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores oxygen as needed during its normal reduction and oxidation process. The TWC releases oxygen as needed during its normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor's voltage levels.
Whenever the voltage levels of the post-catalyst (2) oxygen sensor nears the voltage levels that of the pre-catalyst (1) oxygen sensor, the catalysts efficiency is degraded.
Stepped or staged testing levels allow the control module to statistically filter test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless directed by the electronic service information
Three stages are used in order to monitor catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then looks at the inputs from the pre and post 02 sensors more closely before determining if the catalyst is indeed degraded. This further statistical processing is done to increase the accuracy of oxygen storage capacity type monitoring. Failing the first (stage 0) or second (stage 1) test Does Not indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.
Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an Aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.