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 powertrain control module (PCM) 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 PCM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The PCM 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.

The HO2S 2 signal activity as compared to the HO2S 1 signal activity indicates that the TWC converter oxygen storage capacity is less than 0.08 grams.

DTC P0420 is a Type B DTC.

DTC P0420 is a Type B DTC.

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

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, 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.

2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. 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

•	Exhaust flow restrictions--Refer to Restricted Exhaust

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

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

•	Leaks--Refer to Exhaust Leakage

•	Physical damage

•	Loose or missing hardware

•	Properly torqued HO2S

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

3. 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.

4. Engine running at normal operating temperatures, increase the engine speed to 2,500-3,000 RPM for 3 minutes. With the engine at idle, observe the scan tool HO2S 2 parameter for 1 minute. The HO2S 2 voltage should not be actively switching above and below 450 mV.

⇒	If the HO2S 2 voltage is actively switching, replace the TWC converter.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.