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

Learn how to diagnose and fix P0420 code for GMC Sierra T1XX. Step-by-step guide with diagnostic procedures and repair instructions.

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts.
• Ensure the vehicle is on a level surface, and use jack stands if necessary for access.

2. Inspect and Replace Oxygen Sensors

1. Remove Upstream Oxygen Sensor:

   • Locate the upstream oxygen sensor (before the catalytic converter).
   • Use an oxygen sensor socket to unscrew and remove it.

2. Install New Upstream Oxygen Sensor:

   • Apply anti-seize compound to the threads of the new sensor.
   • Install the new sensor and torque it to the manufacturer’s specifications (typically around 30 ft-lbs).

3. Remove Downstream Oxygen Sensor:

   • Locate the downstream oxygen sensor (after the catalytic converter).
   • Repeat the removal process with the oxygen sensor socket.

4. Install New Downstream Oxygen Sensor:

   • Apply anti-seize compound and install the new sensor, torquing it to the specified value.

3. Inspect and Replace Catalytic Converter (if needed)

1. Inspect Catalytic Converter:

   • Check for signs of damage, such as dents, rust, or excessive heat discoloration.
   • If blockage is suspected, perform a backpressure test.

2. Remove Catalytic Converter:

   • Disconnect exhaust pipes and any brackets holding the catalytic converter in place.
   • Remove bolts using a socket set and lift the converter out.

3. Install New Catalytic Converter:

   • Position the new catalytic converter in place, making sure to align it properly with the exhaust system.
   • Secure it with bolts and torque them to manufacturer specifications.
   • Reconnect any exhaust piping and brackets.

4. Clear Codes and Test Drive

1. Reconnect the battery.
2. Use the OBD-II scanner to clear the P0420 code.
3. Perform a test drive to ensure the check engine light does not reappear and the vehicle operates normally.