Important: A new converter with less than 100 miles on it may set P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

A 3-way catalytic converter (TWC) controls emissions by converting hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed idle conditions follow the listed driving conditions the Catalyst System Efficiency test will run. The engine control module (ECM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific idle conditions. The ECM compares the pre and post catalyst oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

1. Driving Conditions

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DTCs P0010, P0011, P0013, P0014, P0016, P0017, P0018, P0019, P0030, P0036, P0053, P0054, P0059, P0068, P0069, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151-P0155, P0171, P0172, P0174, P0175, P0201-P0208, P0220, P0222, P0223, P0315, P0300, P0301-P0308, P0335, P0336, P0340, P0341, P0351-P0358, P0365, P0366, P0390, P0391, P0401, P0403, P0404, P0405, P0406, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P060D, P1133, P1153, P1174, P1175, P1258, P1400, P1404, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176, P2227, P2228, P2229, P2A00, P2A01, P2A03, or U0107 are not set.

•	The engine has been running for greater than 6 minutes.

•	The vehicle has been driven at greater than 1,000 RPM for more than 1 minute.

•	The vehicle is in Closed Loop.

•	The vehicle has Fuel Trim Learn enabled.

•	The engine coolant temperature (ECT) is between 70-129°C (158-264°F).

•	The barometric pressure (BARO) is greater than 70 kPa.

•	The intake air temperature (IAT) is greater than -20°C (-4°F).

•	The battery voltage is greater than 10.7 volts.

•	The calculated TWC temperature (Calc. TWC Temp.) is between 385-640°C (725-1,184°F).

2. Idle Conditions

•	The vehicle speed is less than 3.2 km/h (2 mph).

•	The throttle position (TP) is less than 2 percent.

•	The short term fuel trim (FT) is between -10 and +10 percent.

•	The engine idle speed is stable.

•	The transmission is in drive.

•	This diagnostic attempts one test during each valid idle period when the above conditions have been met. This diagnostic attempts up to 6 tests during each drive cycle.

DTC P0420 is a Type A DTC.

DTC P0420 is a Type A DTC.

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

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 converter with less than 100 miles on it may set P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.

1. Verify that other DTCs are set.

⇒	If any other DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

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

•	An engine misfire

•	High engine oil or high coolant consumption

•	Retarded spark timing

•	A weak or poor spark

•	A lean fuel mixture

•	A rich fuel mixture

•	A damaged oxygen sensor or wiring harness

•	The catalyst test may abort if the vehicle falls outside the conditions for running the DTC.

•	A catalyst may be temporarily degraded if a fuel with high sulfur content has been used. Drive the vehicle at highway speeds for 10 minutes and retest the converter.

3. Operate the vehicle within the Conditions for Running the DTC.

⇒	If the DTC fails this key cycle, continue with this procedure.

4. Inspect the catalytic converter for the following conditions:

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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.

5. Inspect the exhaust system for the following conditions:

•

Leaks

•	Physical damage

•	Loose or missing hardware

•	Properly torqued HO2S

•	Restrictions

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

6. Inspect the post catalyst HO2S for the following conditions:

•	A grounded wiring harness

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Damage

⇒	If a condition is found, replace the post catalyst HO2S.

⇒	If no physical condition is detected, replace the catalytic converter.

1. Clear the DTCs.
2. Allow the engine coolant temperature to reach 70°C (156°F).

•	Place the gear selector in Park (Neutral for manual).

•	Set the parking brake.

3. Press and hold the service brake.
4. Run the engine for 6 minutes at 2,500 RPM.
5. Allow engine speed to return to idle speed, then place gear selector into Drive for an automatic transmission, or apply clutch pedal for a manual transmission.
6. After 2 minutes verify that DTC P0420 or DTC P0430 have passed or failed this ignition cycle.

⇒

If not passed or failed this ignition cycle, the test aborted resulting in an inconclusive test result. The above test may need to be repeated up to 5 more times during this drive cycle. The first test should be run after the initial warm up period of 6 minutes. Between subsequent tests maintain 2,500 RPM for 2 minutes.

7. If 6 tests have been attempted and the DTC has not run or passed during this key cycle, turn the key to OFF for 30 seconds, then run the procedure a second time.