Diagnostic Instructions

•	Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

•	Review Strategy Based Diagnosis for an overview of the diagnostic approach.

•	Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0420

Catalyst System Low Efficiency

Circuit/System Description

Note

A new converter with less than 161 kilometers (100 miles) on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

A 3-way catalytic converter controls emissions by converting hydrocarbons and CO into CO2, and NOx into nitrogen. The 3 way catalytic converter also stores oxygen. When the driving conditions listed are met, the catalyst system efficiency test will run. The ECM monitors this oxygen storage capacity by comparing the pre- and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific driving conditions. The ECM compares the pre and post-catalyst oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

Conditions for Running the DTC

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DTC P0016, P0017, P0030, P0036, P0053, P0054, P0068, P0090, P0091, P0092, P00C8, P00C9, P00CA, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P013A, P013B, P013E, P0140, P0141, P0171, P0172, P0191, P0192, P0193, P0222, P0223, P0300, P0301, P0302, P0303, P0304, P0335, P0336, P0340, P0341, P0365, P0366, P0443, P0506, P0507, P0606, P0697, P06A3, P16F3, P2101, P2122, P2123, P2127, P2128, P2135, P2138, P2227, P2228, P2229, P2230, P2270, P2271, P228C, P228D, P2A00 is not set.

•	The engine speed is greater than 1125 RPM for 20 s since end of last idle period.

•	The vehicle is in Drive on an automatic transmission vehicle.

•	The vehicle speed is less than 1.2 MPH.

•	The battery voltage is greater than 11 V.

•	Closed Loop fueling is enabled.

•	The Intake Air Temperature (IAT) is greater than -20°C (-4°F).

•	The calculated exhaust mass gas flow is between 2-12 g/s, and stable.

•	The calculated catalyst temperature is between 400-700°C (852-1,382°F), and stable.

•	The Barometric Pressure (BARO) is greater than 70 kPa (10.2 psi).

•	The Engine Coolant Temperature (ECT) is between 40-140°C (104-284°F).

•	The diagnostic attempts one test during each period when the above conditions have been met and may attempt up to 8 tests during each drive cycle.

Conditions for Setting the DTC

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

Action Taken When the DTC Sets

DTC P0420 is a Type A DTC.

Conditions for Clearing the MIL/DTC

DTC P0420 is a Type A DTC.

Reference Information

Electrical Information Reference

•	Circuit Testing

•	Connector Repairs

•	Testing for Intermittent Conditions and Poor Connections

•	Wiring Repairs

•	Heated Oxygen Sensor Wiring Repairs

DTC Types Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Refer to Three-Way Catalytic Converter Damage Caution.

Refer to Heated Oxygen Sensor Resistance Learn Reset Caution.

Note

A new converter with less than 161 kilometers (100 miles) on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

1.	If any other DTCs are set, perform those diagnostics first.

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

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An engine misfire

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High engine oil or high coolant consumption

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Retarded spark timing

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A weak or poor spark

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A lean fuel mixture

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A rich fuel mixture

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A damaged oxygen sensor or wiring harness

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

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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.

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

5.	Inspect the catalytic converter for the following conditions:

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Dents

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A severe discoloration caused by excessive temperatures

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Road damage

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An internal rattle caused by damaged catalyst subset

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Restrictions

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If a condition is found, replace the catalytic converter

6.	Inspect the exhaust system for the following conditions:

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Leaks

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Physical damage

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Loose or missing hardware

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Properly torqued B52B HO2S

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Restrictions

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If a condition is found, repair the exhaust system.

7.	Inspect the post catalyst B52B HO2S for the following conditions:

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A grounded wiring harness

•

Damage

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If a condition is found, replace the post catalyst B52B HO2S.

•

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

Repair Instructions

Note

A new converter with less than 161 kilometers (100 miles) on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 h may correct the condition.

•	Restricted Exhaust

•	Exhaust Leakage

•	Catalytic Converter Replacement (LAU/A28NER) Catalytic Converter Replacement (LBY/A20DTR) Catalytic Converter Replacement (LLU/A16LET) Catalytic Converter Replacement (LBS/A20DTH) Catalytic Converter Replacement (LHU/A20NFT) Catalytic Converter Replacement (LHU/A20NFT - AWD)

•	Heated Oxygen Sensor Replacement - Sensor 2

Repair Verification

1.	Clear the DTCs.

2.	Operate the vehicle within the Conditions for Running and Setting for at least 10 minutes

3.	Stop the vehicle.

4.	Engine OFF, allow the ECM to power down.

5.	Operate the vehicle within the Conditions for Running and Setting for at least 10 minutes DTC P0420 is to be run and disappear.