how to fix P0420 code on 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 tools and parts.
• Disconnect the battery to prevent electrical shorts during the repair.
• Allow the vehicle to cool down to avoid burns.

2. Inspect and Replace Oxygen Sensors

• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors.
  2. Unplug the electrical connectors from both sensors.
  3. Use a socket wrench to remove the oxygen sensors from the exhaust system.
  4. Apply anti-seize compound to the threads of the new sensors.
  5. Install the new oxygen sensors and torque to manufacturer specifications (typically around 30 lb-ft).
  6. Reconnect the electrical connectors.

3. Replace the Catalytic Converter (if necessary)

• Sub-steps:
  1. Remove the exhaust pipes leading to the catalytic converter using a socket set.
  2. Support the catalytic converter and remove any bolts securing it to the exhaust manifold and exhaust system.
  3. Install the new catalytic converter, ensuring proper alignment and sealing.
  4. Torque all bolts to the manufacturer's specifications (usually around 25 lb-ft).
  5. Reconnect the exhaust pipes and ensure all connections are tight.

4. Reset the Engine Control Module (ECM)

• Sub-steps:
  1. Reconnect the battery.
  2. Use the OBD-II scanner to clear the P0420 code from the ECM.
  3. Start the engine and allow it to reach operating temperature.