GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2009 🢒 G5 🢒 DTC 🢒 DTC P0420

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 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 driving conditions are met, the Catalyst System Efficiency test will run. The powertrain control module (PCM) 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 PCM 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
    • DTCs P0300, P0301, P0302, P0303, P0304, P2096, P2097, P2177, P2178, P2187, P2188 are not set.
    • The engine speed is between 1,320-2,480 RPM.
    • The engine load is between 21-68 percent.
    • The vehicle is in Closed Loop.
    • The calculated exhaust mass gas flow is between 10-42 g/sec, and stable.
    • The calculated catalyst temperature is between 525-750°C (977-1,382°F), and stable.
    • The ambient temperature is more than -15°C (+5°F).
    • The rear HO2S has exceeded the dew point for more than 60 seconds.
    • This diagnostic attempts one test during each period when the above conditions have been met. This diagnostic attempts up to 4 tests during each drive cycle.

Conditions for Setting the DTC
    • The PCM has determined the catalyst efficiency has degraded below a calibrated threshold.
    • This diagnostic may conclude in 6 test attempts which will require at least 2 drive cycles. However, this diagnostic may require as many as 8 test attempts. Depending on driving conditions, more than 2 drive cycles may be required. Each test attempt may conclude within approximately 1 minute.

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

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

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

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 hour 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:
    3. • 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.
    4. ⇒If the DTC fails this key cycle, continue with this procedure.
  4. Inspect the catalytic converter for the following conditions:
    5. • 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:
    6. • 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:
    7. • A grounded wiring harness
    • Damage
         ⇒If a condition is found, replace the post catalyst 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 hour may correct the condition.

    •  Catalytic Converter Replacement
    •  Heated Oxygen Sensor Replacement - Sensor 2
    •  Exhaust Leakage
    •  Restricted Exhaust

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 should run and pass.