K 2500 Yukon 4WD V8-6.0L VIN U (2000)
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 converter.
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 heated oxygen sensors. The pre-sensors produce an output signal which indicates
the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. 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 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 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 degrees hydrocarbon conversion efficiency, shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor
(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
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HO2S contamination
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Alternative fuels
Exhaust system leaks may cause the following:
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Prevent a degraded catalyst from failing the diagnostic
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Cause a false failure for a normally functioning catalyst
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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
