The system uses a three-way catalytic converter in order to control the emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The VCM has the capability to monitor this process using the HO2S (Bank 1, Sensor 1). The HO2S (Bank 1, Sensor 3), located in the exhaust downstream of the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S (Bank 1, Sensor 3) signal will be far less active than that produced by the HO2S (Bank 1, Sensor 2). If a problem exists which causes the VCM to detect an excessive HO2S (Bank 1, Sensor 3) activity outside of an acceptable range for an extended period of time, the VCM sets the DTC P0420. This DTC indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable. This is a type A DTC.

Converter Warm Up Test

Converter Warm UP Test Passed

The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.

Difficulty running the OBD II status DTC P0420 test may be encountered in areas where the test conditions cannot be maintained easily, especially in urban areas.

The numbers below refer to the step numbers in the diagnostic table.

3. This table includes checks for conditions that can cause the three-way catalytic converter efficiency to appear degraded. Inspect and repair exhaust system as necessary. Also inspect the HO2S (Bank 1, Sensor 3) pigtail and engine harness for any damage that can cause an intermittent fault in HO2S (Bank 1, Sensor 3) sensor signal HIGH and LOW circuits.

8. Notice: In order to avoid damaging the replacement three-way catalytic converter, correct the engine misfire or mechanical fault before replacing the three-way catalytic converter.

   If the three-way catalytic converter needs to be replaced, make sure that the following conditions are not present to caused the catalyst to be damaged:

•

Misfire

•	High engine oil consumption or coolant consumption

•	Retarded spark timing or weak spark.