K 1500 Yukon Denali AWD V8-6.0L VIN U (2002)
If the HO2S pigtail wiring, connector, or terminal are damaged, replace the entire HO2S assembly. Do not attempt to repair the wiring, the connector, or
the terminals. In order for the sensor to function properly, the sensor must be provided with a clean air reference. This clean air reference is obtained by
way of the HO2S 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 HO2S 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 TWC. The catalyst
promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and CO into harmless water vapor and carbon
dioxide. The catalyst also converts NOx to 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 HO2S. The pre-sensors produce an output signal which indicates the amount of
oxygen present in the exhaust gas entering the TWC. The post sensor produces an output signal which indicates the oxygen storage capacity of the
catalyst. This in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the pre-HO2S signal is far
more active than that produced by the post-HO2S.
In addition to catalyst monitoring, the post-HO2S 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 HO2S are installed before and after
the 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 catalysts 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 HO2S. 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 correct
oxygen sensor voltages like those shown in the post-catalyst HO2S outputs graphic.
The catalyst monitor diagnostic is sensitive to the following conditions:
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Exhaust leaks
^
HO2S contamination
^
Alternative fuels
Exhaust system leaks may cause the following:
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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 prevent the TWC
diagnostic from functioning properly.
Three-Way Catalyst Oxygen Storage Capacity
