Alero L4-144 2.4L DOHC VIN T SFI (2000)
Catalytic Converter: Description and Operation
Catalyst Monitor Diagnostic Operation
The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the 3-Way
Catalyst (TWC). The 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 capacity of the catalyst to store and release oxygen generally degrades. The
OBD II 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 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 HO2S 2 must be at operating temperature in order to achieve correct
oxygen sensor voltages like those shown in the post-catalyst HO2S 2 Outputs graphic.
The catalyst monitor diagnostic is sensitive to the following conditions:
^
Any exhaust leaks
^
Any contamination of the HO2S 2
^
Any alternative 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 following contaminants that may be encountered:
^
Phosphorus
^
Lead
^
Silica
^
Sulfur
The presence of these contaminants prevents the TWC diagnostic from functioning properly.
