how to fix P0420 code in GMC Sierra T1XX (2019-Present)

Learn how to diagnose and fix P0420 code related to catalytic converter efficiency in GMC Sierra T1XX. Step-by-step guide with tools and procedures.

Solution

1. Preparation

• Gather necessary tools and equipment.
• Park the vehicle on a level surface and engage the parking brake.
• Disconnect the negative battery terminal to ensure safety.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil.
• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  2. Apply penetrating oil to the sensor threads if they are rusted.
  3. Using the O2 sensor socket, carefully remove both sensors.
  4. Install the new sensors by threading them into place and tightening them to the manufacturer's specified torque (typically 30 lb-ft).
  5. Reconnect the electrical connectors for both sensors.

3. Inspect and Replace Catalytic Converter

• Tools Required: Socket set, torque wrench, exhaust pipe cutter, gaskets.
• Sub-steps:
  1. Remove any heat shields or brackets obstructing access to the catalytic converter.
  2. Cut the exhaust pipe before and after the catalytic converter using an exhaust pipe cutter.
  3. Remove the old catalytic converter.
  4. Install the new catalytic converter, ensuring proper alignment with the exhaust system.
  5. Use new gaskets and tighten all bolts to the manufacturer's specified torque (typically 40-50 lb-ft).
  6. Reinstall any heat shields or brackets removed during the process.

4. Clear Codes and Test

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Reconnect the negative battery terminal.
  2. Start the engine and let it reach operating temperature.
  3. Use the OBD-II scanner to clear the P0420 code from the PCM.
  4. Perform a test drive to ensure the check engine light does not reappear.