how to fix P0420 code on my 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 required tools and parts.
• Disconnect the negative battery terminal.
• Allow the vehicle to cool down completely before working on the exhaust system.

2. Inspect and Replace Oxygen Sensors

1. Remove the Upstream Oxygen Sensor:

   • Use an oxygen sensor socket to unscrew the sensor from the exhaust manifold.
   • Disconnect the electrical connector.

2. Replace with New Sensor:

   • Apply anti-seize compound to the threads of the new oxygen sensor.
   • Install the new sensor and torque to the manufacturer's specifications (typically around 30 ft-lbs).
   • Reconnect the electrical connector.

3. Repeat for Downstream Oxygen Sensor:

   • Follow similar steps to remove and replace the downstream oxygen sensor located before the catalytic converter.

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove Exhaust Components:

   • Loosen and remove bolts securing the catalytic converter to the exhaust pipe and exhaust manifold.
   • Carefully detach any clamps or hangers securing the converter.

2. Install New Catalytic Converter:

   • Position the new catalytic converter and align with exhaust flanges.
   • Torque all bolts to the manufacturer’s specifications, ensuring a secure fit.

3. Reattach Exhaust Components:

   • Reconnect any previously removed clamps or hangers.

4. Reconnect Battery and Clear Codes

• 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.