GMC Sierra T1XX (2019-Present) engine code 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 necessary tools and parts.
• Ensure the vehicle is on a level surface and the engine is cool.
• Disconnect the battery to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, multimeter.
• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  2. Using the O2 sensor socket, remove the sensors by turning counter-clockwise.
  3. Inspect the connectors and wiring for damage or corrosion.
  4. If faulty, replace with new sensors, ensuring they are torqued to the manufacturer's specifications (typically around 30 lb-ft).
  5. Reconnect the battery.

3. Inspect and Replace Catalytic Converter

• Tools Required: Socket set, exhaust pipe cutter (if necessary), gasket scraper, torque wrench.
• Sub-steps:
  1. Raise the vehicle using jack stands and remove any underbody shields if applicable.
  2. Disconnect the exhaust pipes from the catalytic converter.
  3. If the catalytic converter is bolted, remove the bolts using a socket set. If welded, use an exhaust pipe cutter to remove.
  4. Clean the exhaust flange surfaces with a gasket scraper.
  5. Install a new catalytic converter, making sure to use new gaskets as needed. Torque bolts to manufacturer specifications.
  6. Reconnect exhaust pipes and any underbody shields.

4. Clear Codes and Test Drive

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Use the OBD-II scanner to clear the P0420 code from the system.
  2. Start the engine and let it idle for a few minutes.
  3. Conduct a test drive to ensure the vehicle operates normally and check for any reoccurrence of the check engine light.