A 3-way catalytic converter (TWC) 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 engine control module (ECM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. The HO2S 2 produces an output signal that the ECM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The ECM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The ECM 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.

This diagnostic attempts one test during each valid idle period once the above conditions have been met. This diagnostic attempts up to 12 tests during each drive cycle.

The ECM determines that the efficiency of the catalyst has degraded below a calibrated threshold.

DTC P0420 is a Type A DTC.

DTC P0420 is a Type A DTC.

Inspect for the following conditions, which may cause a catalytic converter to degrade:

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Engine idling, observe the DTC information with a scan tool. Verify there are no HO2S or misfire DTCs set.

⇒	If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnosis the applicable DTC, before proceeding with this diagnostic procedure.

2. Verify the following conditions do not exist with the catalytic converter:

•

Dents

•	A severe discoloration caused by excessive temperatures

•	Road damage

•	An internal rattle caused by damaged catalyst substrate

•	Restrictions

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

3. Verify the following conditions do not exist with the exhaust system:

•

Leaks

•	Physical damage

•	Loose or missing hardware

•	Properly torqued HO2S

⇒	If a condition is found, repair the exhaust system.

4. Verify the following conditions do not exist with the HO2S 2:

•	A grounded wiring harness

•

Damage

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

5. If no physical condition is detected and the HO2S 2 is as active as the HO2S 1 after operating the engine at 1,500 RPM for 1 minute and then returning to a stabilized idle, then replace the catalytic converter.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.