GMC Sierra T1XX (2019-Present) check engine light P0420 how to fix

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

Solution

1. Preparation

• Gather tools and parts.
• Disconnect the negative battery terminal to avoid electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: Oxygen sensor socket, ratchet, and torque wrench.
• Remove the upstream oxygen sensor:
  1. Locate the upstream (pre-catalytic) oxygen sensor.
  2. Use the oxygen sensor socket and ratchet to unscrew the sensor.
  3. Inspect the sensor for damage or contamination.
  4. Install the new sensor and torque to manufacturer's specifications (typically around 30 ft-lbs).
• Repeat for the downstream oxygen sensor if testing indicated it is faulty.

3. Inspect and Repair Exhaust System

• Tools Required: Wrenches, jack stands, and safety glasses.
• Visually inspect the exhaust system:
  1. Look for cracks, rust, or holes in the exhaust pipes and the catalytic converter.
  2. If any damage is found, replace the affected components.
  3. Ensure all connections are secure and free of leaks.

4. Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, cutting tools (if required), and torque wrench.
• Remove the old catalytic converter:
  1. Disconnect the exhaust pipes from the catalytic converter, using a cutting tool if necessary.
  2. Remove any brackets or supports securing the converter.
  3. Install the new catalytic converter and secure it using new gaskets.
  4. Torque all bolts to specified values.

5. Clear Codes and Test Drive

• Tools Required: OBD-II scanner.
• Reconnect the battery terminal.
• Use the OBD-II scanner to clear the P0420 code.
• Test drive the vehicle to ensure the check engine light does not reappear.