In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and the CO that is present in the exhaust gases. The oxidation process converts the HC and CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors (HO2S 1 and HO2S 2). The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gases entering the TWC converter. The HO2S 2 produces an output signal that indicates the oxygen storage capacity of the catalyst in the TWC converter. The oxygen storage capacity of the catalyst is an indicator of the converter ability to convert exhaust gases efficiently. The HO2S 1 output signal will be far more active than the HO2S 2 output signal when the converter catalyst is operating efficiently. A DTC P0420 will set if the PCM detects a level of HO2S 2 activity that indicates the catalyst is no longer operating efficiently.

Check for any of the following conditions:

A TWC converter failure may be caused by any of the following conditions:

If a DTC P0420 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0420 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0420 appears in the Not Ran Since Code Cleared list, the P0420 diagnostic has not yet run. When the DTC P0420 does not appear in the Not Ran Since Code Cleared list, the P0420 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.

If a DTC P0420 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.

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

1. The OBD System Check prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

3. This step verifies whether the malfunction is presently occurring.

4. This step checks whether the malfunction that caused the DTC P0420 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool Clear DTC Information function, the DTC P0420 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0420 appears in the Not Ran Since Code Cleared list, the P0420 diagnostic has not yet run. When DTC P0420 does not appear in the Not Ran Since Code Cleared list, the P0420 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the P0420 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.

8. In order to avoid repeated replacement of the catalytic converter, inspect for the cause of the converter failure and repair as necessary. Refer to Diagnostic Aids.