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 tools and parts needed.
• Disconnect the battery to avoid electrical shorts.
• Allow the engine to cool before working on the exhaust system.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Remove the upstream oxygen sensor:
  1. Locate the upstream O2 sensor (usually before the catalytic converter).
  2. Use the O2 sensor socket to remove it.
  3. Install the new O2 sensor and tighten to the manufacturer’s specifications (usually around 30 ft-lbs).
• Repeat the process for the downstream oxygen sensor.

3. Inspect and Repair Exhaust Leaks

• Tools Required: Exhaust pipe cutter, welding kit or exhaust clamps.
• Inspect the exhaust system for leaks:
  1. Start the engine and listen for hissing sounds around the joints.
  2. If any leaks are found, repair them by either welding or using exhaust clamps as appropriate.

4. Replace Catalytic Converter (if needed)

• Tools Required: Socket set, torque wrench, exhaust gasket.
• Remove the catalytic converter:
  1. Disconnect any brackets or bolts securing the catalytic converter to the exhaust system.
  2. Cut the exhaust pipe if necessary for easier removal.
  3. Install the new catalytic converter with new gaskets and tighten bolts to specifications.

5. Clear Codes and Test

• Reconnect the battery.
• Use the OBD-II scanner to clear the P0420 code from the system.
• Start the vehicle and allow it to reach operating temperature.