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

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

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors (if necessary)

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Remove the upstream oxygen sensor (before the catalytic converter) using the O2 sensor socket.
  • Sub-steps:
    • Disconnect the electrical connector from the sensor.
    • Use the O2 sensor socket to unscrew and remove the sensor.
• Install the new oxygen sensor and torque it to the manufacturer's specifications (usually around 30-50 lb-ft).
• Reconnect the electrical connector.

3. Inspect and Replace the Catalytic Converter (if necessary)

• Tools Required: Pipe cutter or saw, socket set, torque wrench.
• Remove the exhaust pipes connected to the catalytic converter.
  • Sub-steps:
    • Use a pipe cutter or saw to remove any bolts or clamps securing the pipes.
    • Carefully detach the catalytic converter from the exhaust system.
• Install the new catalytic converter, ensuring proper alignment with the exhaust pipes.
  • Torque the bolts connecting the catalytic converter to the manufacturer's specifications (typically 20-30 lb-ft).

4. Clear Codes and Test Drive

• Reconnect the battery.
• Use the OBD-II scanner to clear the P0420 code.
• Perform a test drive to monitor vehicle performance and ensure the check engine light does not reappear.