GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Saturn 🢒 2009 🢒 SKY 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 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 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.

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.

Conditions for Running the DTC

Driving Conditions

    • DTCs P0030, P0036, P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0220, P0300, P0315, P0326, P0327, P0336, P0340, P0341, P0442, P0446, P0452, P0453, P0455, P0496, P0502, P0506, P0507, P0601, P0602, P1133, P1516, P1621, P2135, P2138, P2176 are not set.
    • The engine has been running for more than 10 minutes.
    • The vehicle has been driven at more 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 70-125°C (156-257°F).
    • The barometric pressure (BARO) is more than 70 kPa.
    • The catalytic converter calculated temperature is greater than or equal to 450°C (842°F).
    • The intake air temperature (IAT) is between -20 and +85°C (-4 and +185°F).
    • The battery voltage is more than 11 volts.

Idle Conditions

    • The vehicle speed is less than 3.2 km/h (2 mph).
    • For automatic transmissions, the transmission is in Reverse, Drive, or Low.
    • The throttle position (TP) is 2 percent or less.
    • The short term fuel trim (FT) is between -10 and +10 percent.
    • 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.

Conditions for Setting the DTC
    • The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.
    • This diagnostic may conclude in one test attempt. However, this diagnostic may require as many as 18 test attempts, which would require at least 3 drive cycles. Each test attempt may conclude within approximately 1 minute.

Action Taken When the DTC Sets

DTC P0420 is a Type B DTC.

Conditions for Clearing the MIL/DTC

DTC P0420 is a Type B 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
  1. Verify that no other DTCs are set.
    2. ⇒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.
    3. ⇒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.
    8. ⇒If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.

Circuit/System Testing

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:
    2. • 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:
    3. • 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:
    4. • 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:
    5. • A grounded wiring harness
    • Damage
         ⇒If a condition is found, replace the post catalyst HO2S.
  5. If all physical conditions test normal, test or replace the catalytic converter.

Repair Instructions

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.

    •  Exhaust Manifold Replacement
    •  Catalytic Converter Replacement

Repair Verification
  1. Verify that no other DTCs are set.
    2. ⇒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.
    3. ⇒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.
    8. ⇒If no decision has been made, repeat steps 2-6 until the Test Result parameter indicates a Pass or Fail.