Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

A 3-way catalytic (TWC) converter controls 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 the CO that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The control module monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. This HO2S 2, also referred to as the catalyst monitor sensor, produces an output signal that the control module uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The control module monitors the efficiency of the catalyst by allowing the catalyst to heat, then waits for a stabilization period while the engine is idling. The control module then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold. If the control module detects the degraded condition, DTC P0420 sets.

Caution: Refer to Road Test Caution in the Preface section.

Scan Tool Reference

Scan Tool Data List

Electrical Information Reference

Operate the vehicle within the Conditions for Running the DTC.

Notice: Refer to Heated Oxygen Sensor Resistance Learn Reset Notice in the Preface section.

Notice: Refer to Three-Way Catalytic Converter Damage Notice in the Preface section.

Important: A new catalyst may fail this test due to out-gassing of the internal matting. If this occurs, operate the vehicle at highway speeds for approximately one hour and retest.

1. Inspect the catalytic converter for the following conditions:

•

Dents

•	A severe discoloration caused by excessive temperatures

•	Road damage

•	An internal rattle caused by damaged catalyst substrate

•	Restrictions

⇒	If a condition is found, then replace the catalytic converter.

2. Inspect the exhaust system for the following conditions:

•

Leaks

•	Physical damage

•	Loose or missing hardware

⇒	If a condition is found, then replace the exhaust system or components, as necessary.

3. Inspect the HO2S 2 sensor for proper torque.

⇒	If the HO2S 2 sensor does not have proper torque, correctly torque the sensor.

4. Inspect the HO2S 2 sensor and circuits for the following conditions:

•	Grounded or chafing wiring harness

•

Damage

⇒	If a condition is found, then replace the HO2S 2.

⇒	If no physical condition is detected, perform the following:

-	Clear all DTCs.

-	Operate the vehicle under the Conditions for Running the DTC.

⇒	If DTC P0420 runs and fails, replace the converter.

Important: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Important: A new catalyst may fail this test due to out-gassing of the internal matting. If this occurs, operate the vehicle at highway speeds for approximately one hour and retest.

1. Clear all DTCs.

Important: DO NOT touch the accelerator while a catalyst test is in progress.

2. Operate the vehicle within the Conditions for Running the DTC.
3. Verify if DTC P0420 has passed or failed this ignition cycle with the scan tool.
4. If 6 tests have been attempted and the DTC has not run or passed during this key cycle, turn the ignition to OFF for 30 seconds, then run the test a second time.
5. A maximum of 6 tests per key cycle will run if each test is a combination of passed, failed, or aborted tests.
6. After a code clear, this diagnostic may conclude in as few as one test attempt. However, this diagnostic may require as many as 18 test attempts which would require at least 3 drive cycles.