GMC Sierra T1XX (2019-Present) engine light on with 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 tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet and extensions, torque wrench, penetrating oil.
• Sub-steps:
  1. Locate and remove the upstream oxygen sensor using the O2 sensor socket.
  2. Inspect the wiring and connector for damage; replace if necessary.
  3. Install the new oxygen sensor and torque it to the manufacturer’s specifications (typically 30 lb-ft).
  4. Repeat the process for the downstream oxygen sensor.

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

• Tools Required: Socket set, torque wrench, cutting tool (if welding is required), exhaust clamps.
• Sub-steps:
  1. Inspect the catalytic converter for any physical damage or blockage.
  2. If replacement is necessary, remove the bolts or clamps securing the catalytic converter to the exhaust system.
  3. Install the new catalytic converter, ensuring a proper fit and sealing.
  4. Torque all bolts to the manufacturer’s specifications and check for leaks.

4. Clear Error Codes

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Reconnect the negative battery terminal.
  2. Use the OBD-II scanner to clear the P0420 code from the vehicle’s computer.