Metro L4-079 1.3L VIN 2 MFI (2001)
Powertrain Control Module: Description and Operation
Primary Sys. Based Diagnostics
General Description
The Powertrain Control Module (PCM) uses certain diagnostic strategies known as primary system based diagnostics that evaluate the various primary
system operations. The primary system based diagnostics also evaluate the various primary system operations affect on vehicle emissions. Some of the
primary system based diagnostics are listed below with a brief functional description of the diagnostics involved.
Catalyst Monitor Diagnostic Operation
The OBD II catalyst monitor diagnostic measures the oxygen storage capacity of the Three-way Catalytic Converter (TWC). Heated oxygen sensors are
installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the powertrain control module to
determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity
to store and release oxygen is generally degraded. The OBD II catalyst monitor diagnostic is based on a correlation between the conversion efficiency of
the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst (e.g. 95 percent hydrocarbon conversion efficiency) will show a
relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S 2). A degraded catalyst (65 percent hydrocarbon conversion) will show
greatly increased activity in the output voltage from the post catalyst HO2S.
The post-catalyst HO2S is used to measure the oxygen storage/release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a
good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve
the 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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Alternate fuels
Exhaust system leaks may cause any of the following results:
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A false failure for a normally functioning (good) catalyst.
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Prevent a degraded (bad) catalyst from failing the catalyst monitor diagnostic.
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Prevent the catalyst monitor diagnostic from running.
The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.
Three-Way Catalyst Oxygen Storage Capacity
The Three-way Catalytic Converter (TWC) catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the
pre-catalyst HO2S and post-catalyst HO2S oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly
less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during its normal reduction and oxidation process. The
control module will calculate the oxygen storage capacity using the difference between the pre catalyst and post catalyst oxygen sensors voltage levels. If
the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.
Stepped or staged testing levels allow the powertrain control module to statistically filter test information. This prevents falsely passing or falsely failing
the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen
sensor activity should not be used to determine oxygen storage capacity unless directed by the service manual.
A two stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in
order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S sensors more closely in
order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing
the first stage test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuel's sulfur content could be very high.
Aftermarket HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false
pass or a false fail of the catalyst monitor diagnostic. An aftermarket catalytic converter that does not contain the same amount of cerium as the original
catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and
the conversion efficiency of the TWC.
Catalyst Monitor (Good Catalyst)
