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

Learn how to diagnose and fix P0420 exhaust system 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 negative battery terminal to prevent electrical shorts.

2. Inspect and Repair Exhaust Leaks

• Tools Required: Wrench set, exhaust leak detection solution.
• Inspect all exhaust joints and welds for leaks.
• Use the exhaust leak detection solution to identify small leaks.
• Repair or replace any damaged sections of the exhaust system.

3. Replace Oxygen Sensors (if necessary)

• Tools Required: O2 sensor socket, ratchet, and extension.
• Locate the upstream and downstream oxygen sensors.
• Disconnect the electrical connectors from the sensors.
• Use the O2 sensor socket to unscrew the faulty sensors.
• Install new sensors by threading them into the exhaust manifold and catalytic converter.
• Reconnect the electrical connectors.

4. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Wrench set, jack and jack stands (or lift), exhaust gasket.
• Safely lift the vehicle and support it securely.
• Disconnect the exhaust pipes from the catalytic converter using a wrench.
• Remove any mounting bolts that secure the catalytic converter.
• Replace with a new catalytic converter, ensuring proper alignment.
• Reattach exhaust pipes and tighten bolts to the manufacturer’s torque specifications.

5. Reset the Engine Control Module (ECM)

• Tools Required: OBD-II scanner.
• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 code from the ECM.