how to fix P0420 code on 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 necessary tools and parts.
• Disconnect the negative battery terminal to ensure safety.
• Allow the vehicle to cool down before performing any work on the exhaust system.

2. Inspect and Repair Exhaust Leaks

• Tools Required: Socket set, exhaust leak detector (if available)
• Inspect all joints, gaskets, and pipes in the exhaust system for leaks.
• If leaks are found, replace damaged gaskets or tighten loose connections.
• For significant damage, replace the affected exhaust component (e.g., pipe or muffler).

3. Replace Oxygen Sensors (if necessary)

• Tools Required: O2 sensor socket, torque wrench
• Locate the upstream and downstream oxygen sensors on the exhaust system.
• Disconnect the electrical connectors from the sensors.
• Use the O2 sensor socket to unscrew the sensors from their mounting locations.
• Install the new sensors, ensuring to torque them to the manufacturer's specifications (typically around 30-35 ft-lbs).
• Reconnect the electrical connectors securely.

4. Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, cutting tool (if applicable), torque wrench
• If the catalytic converter is confirmed to be faulty after inspection, remove it by unbolting it from the exhaust system.
• If welded in place, cut the old catalytic converter out carefully.
• Install the new catalytic converter, ensuring proper alignment and sealing.
• Torque all bolts to the manufacturer's specifications, typically around 30-40 ft-lbs.

5. Clear Codes and Test Drive

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 code.
• Start the engine and allow it to reach operating temperature.
• Test drive the vehicle under various conditions to ensure the issue is resolved.