A 3-way catalytic (TWC) converter controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). 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.

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

1. Driving Conditions

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DTCs P0030, P0036, P0053, P0054, P0059, P0068, P0101, P0102, P0103, P0106, P0107, P0108, 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, P0401, P0403, P0404, P0405, P0406, P0411, P0412, P0418, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P060D, P1101, P1133, P1153, P1174, P1175, P1258, P1400, P1404, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176, P2430, P2431, P2432, P2433, P2440, P2444, P2A00, P2A01, P2A03 are not set.

•	The engine has been running for greater than 2 minutes.

•	The vehicle is in Closed Loop.

•	The vehicle has Fuel Trim Learn enabled.

•	The vehicle has been driven at greater than 1,000 RPM for greater than 16 seconds since the previous idle period.

•	The mass air flow (MAF) has been greater than 12 g/s for greater than 30 seconds.

•	The engine coolant temperature (ECT) is between 40-130°C (104-266°F).

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

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

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

•	The calculated TWC temperature (Calc. TWC Temp.) is between 400-640°C (752-1,184°F).

•	The battery voltage is greater than 10.9 volts.

2. Idle Conditions

•	The vehicle speed is less than 2 km/h (1 mph).

•	The engine speed has been greater than 1,000 RPM for 16 seconds.

•	The automatic transmission is in Reverse, Drive, or Low.

•	The TP is less than 2 percent.

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

•	The engine speed is less than 1,000 RPM.

•	The engine idle is stable.

•	This diagnostic attempts one test during each valid idle period until a pass or fail decision has been reached when the above conditions have been met. These 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

1. Verify that no other DTCs are set.

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

2. Operate the vehicle within the Conditions for Running the DTC while observing the scan tool TWC Mon. OFF Idle Test Conditions Met parameter. The parameter will change to Yes when the OFF Idle Conditions have been met.

⇒	If not the specified value, operate the vehicle within the OFF Idle Conditions.

3. When the parameter has changed to Yes and the conditions are safe, stop and idle the vehicle in gear.
4. Apply the service brake.
5. Monitor the scan tool TWC Monitor Test State parameter. When the parameter indicates Active, the TWC Mon Test is running.
6. Continue to monitor the parameter. When the parameter indicates Inactive, the TWC Mon Test has completed.
7. When the test has completed, the TWC Monitor Test Count parameter will increment by one and the TWC Monitor Test Result parameter will indicate Pass, Fail, or No Decision.

⇒	If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.

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

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

1. 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

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

•	A catalyst test 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.

2. 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.

3. 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.

4. 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.

5. If all physical conditions test normal, test or replace the catalytic converter.

1. Verify that no other DTCs are set.

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

2. Operate the vehicle within the Conditions for Running the DTC while observing the scan tool TWC Mon. OFF Idle Test Conditions Met parameter. The parameter will change to Yes when the OFF Idle Conditions have been met.

⇒	If not the specified value, operate the vehicle within the OFF Idle Conditions.

3. When the parameter has changed to Yes and the conditions are safe, stop and idle the vehicle in gear.
4. Apply the service brake.
5. Monitor the scan tool TWC Monitor Test State parameter. When the parameter indicates Active, the TWC Mon Test is running.
6. Continue to monitor the parameter. When the parameter indicates Inactive, the TWC Mon Test has completed.
7. When the test has completed, the TWC Monitor Test Count parameter will increment by one and the TWC Monitor Test Result parameter will indicate Pass, Fail, or No Decision.

⇒	If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.