Heated Oxygen Sensors (Pre-Catalyst)
The main function of the pre-catalyst heated oxygen sensor (HO2S) is to provide the PCM with exhaust stream information in order to maintain proper fueling to hold emissions within acceptable levels.
These oxygen sensors are always located between the exhaust manifold and the catalytic converter. 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 PCM 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 PCM 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
In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a 3-way catalytic (TWC) converter. The catalyst promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the elements into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx into nitrogen. Catalyst monitor HO2S (Post Catalyst HO2S) are always located downstream of the catalytic converter.
The PCM has the ability to monitor this process using the post catalyst heated oxygen sensors. The pre-sensors produce an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The post 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 pre HO2S signal will be far more active than that produced by the post HO2S.
In addition to catalyst monitoring, the post heated oxygen sensor has a limited role in controlling fuel delivery. If the post HO2S signal indicates a high or low oxygen content for an extended period of time while in a closed loop, the PCM adjusts the fuel delivery slightly in order to compensate.
Catalyst Monitor Diagnostic Operation
The catalyst monitor diagnostic measures oxygen storage capacity of the catalyst converter. In order to do this, the heated sensors are installed before and after the 3-way catalyst (TWC). Voltage variations between the sensors allow the PCM 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 catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.
A good catalyst (e.g., 95 percent hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst heated oxygen sensor (HO2S) signal circuit. A degraded catalyst (65 percent hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.
The post-catalyst HO2S 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 the HO2S must be at operating temperature in order to achieve reliable oxygen sensor voltages like those shown in the Three-Way Catalyst Oxygen Storage Capacity graphic.
The PCM performs the catalyst diagnostic at idle when the conditions for running the diagnostic are met. Refer to Conditions for Running the DTC in DTC P0420 or P0430 . During the catalyst diagnostic the PCM captures the current rear HO2S rich/lean status. The Air Fuel ratio transitions from rich to lean or lean to rich depending on the initial captured rich/lean status. The Air Fuel ratio transitions a second time opposite the first Air Fuel ratio transition. During this diagnostic the scan tool will display HO2S voltages going from full rich to full lean. This condition is normal during this diagnostic.
The catalyst monitor diagnostic is sensitive to the following conditions:
Exhaust leaks
HO2S Contamination
Alternative fuels (Ethanols and Methanols)
Exhaust system leaks may cause the following:
Prevent a degraded catalyst from failing the diagnostic
Cause a false failure for a normally functioning catalyst
Prevent the diagnostic from running