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

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 all necessary tools and parts.
• Disconnect the vehicle's battery to prevent electrical shorts during repairs.
• Ensure the vehicle is on a level surface and secured with jack stands if elevation is required.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, wrench set, multimeter
• Sub-steps:
  1. Locate the upstream (pre-cat) and downstream (post-cat) oxygen sensors.
  2. Disconnect the electrical connectors from both sensors.
  3. Use the O2 sensor socket to remove the sensors from the exhaust manifold and catalytic converter.
  4. Inspect the threads of the sensors; replace if damaged.
  5. Install new oxygen sensors, ensuring they are torqued to manufacturer's specifications (typically 30-40 ft-lbs).
  6. Reconnect the electrical connectors.

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

• Tools Required: Socket set, torque wrench, cutting tool (if necessary), exhaust hanger tool
• Sub-steps:
  1. Confirm the catalytic converter is faulty based on previous diagnostic steps.
  2. Remove any heat shields or clamps that may obstruct access to the catalytic converter.
  3. Disconnect the exhaust pipe from the catalytic converter using appropriate sockets.
  4. If the catalytic converter is welded, carefully cut it out using an appropriate cutting tool.
  5. Install the new catalytic converter, ensuring it fits securely and aligns with the exhaust system.
  6. Reattach any heat shields and clamps, torqueing to manufacturer specifications (typically 15-20 ft-lbs).

4. Clear Codes and Test

• Tools Required: OBD-II scanner
• Sub-steps:
  1. Reconnect the vehicle's battery.
  2. Use the OBD-II scanner to clear any stored trouble codes.
  3. Start the engine and allow it to reach operating temperature.
  4. Drive the vehicle for a short distance to ensure the system monitors complete.