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

Learn how to diagnose and fix P0420 exhaust system issue in GMC Sierra T1XX. Step-by-step guide with diagnostic procedures and repair instructions.

Solution

1. Preparation

• Gather all necessary tools and parts.
• Park the vehicle on a flat surface, engage the parking brake, and allow the engine to cool.
• Disconnect the negative battery terminal to avoid any electrical issues.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet and extensions.
• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors in the exhaust system.
  2. Using the O2 sensor socket, carefully unscrew and remove the upstream O2 sensor.
  3. Install the new upstream O2 sensor, tightening it to the manufacturer’s torque specification (usually around 30 lb-ft).
  4. Repeat the process for the downstream O2 sensor.
  5. Reconnect the battery terminal.

3. Inspect and Replace Catalytic Converter (if needed)

• Tools Required: Exhaust pipe cutter or saw, socket set, torque wrench, penetrating oil.
• Sub-steps:
  1. If the catalytic converter is found to be damaged or clogged, prepare to replace it.
  2. Apply penetrating oil to the bolts connecting the catalytic converter to the exhaust system and let it sit for a few minutes.
  3. Using an exhaust pipe cutter or saw, cut the exhaust pipes to remove the old catalytic converter.
  4. Install the new catalytic converter, aligning it with the exhaust pipes.
  5. Secure the connections and torque to the manufacturer’s specifications.

4. Erase Codes and Test Drive

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Use the OBD-II scanner to erase the P0420 code from the ECU.
  2. Start the vehicle and allow it to reach operating temperature.
  3. Take the vehicle for a test drive to monitor for any reoccurring codes and ensure performance has improved.