Astro Van 2WD V6-4.3L VIN W (2001)
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.
Input Components
The PCM monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking
refers to indicating a fault when the signal from a sensor does not seem reasonable, such as a Throttle Position (TP) sensor that indicates high throttle
position at low engine loads or MAP voltage. The input components may include, but are not limited to, the following sensors:
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The Vehicle Speed Sensor (VSS)
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Mass Air Flow (MAF) sensor
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Intake Air Temperature (IAT) sensor
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Crankshaft Position (CKP) sensor
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Knock Sensor (KS)
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Throttle Position (TP) sensor
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Engine Coolant Temperature (ECT) sensor
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Camshaft Position (CMP) sensor
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Manifold Absolute Pressure (MAP) sensor
In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature to enable
Closed Loop fuel control.
Output Components
Diagnose the output components for the proper response to PCM commands. Components where functional monitoring is not feasible will be monitored
for circuit continuity and out-of-range values if applicable.
Output components to be monitored include, but are not limited to, the following circuits:
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The Idle Air Control (IAC) motor
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The EVAP system
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The electronic transmission controls
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The A/C relay, if so equipped
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The Vehicle Speed Sensor (VSS) output
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The Malfunction Indicator Lamp (MIL) control
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The cruise control enable, if so equipped
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
