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 necessary tools and parts.
• Disconnect the battery to ensure safety.
• Allow the engine to cool completely before starting work.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil (if needed)

• Remove the upstream oxygen sensor:

  1. Locate the upstream O2 sensor (before the catalytic converter).
  2. Use penetrating oil if the sensor is stuck.
  3. Disconnect the electrical connector.
  4. Use the O2 sensor socket to remove the sensor.

• Install the new upstream oxygen sensor:

  1. Apply anti-seize compound to the threads of the new sensor.
  2. Install the new sensor and tighten to the manufacturer’s specified torque (usually around 30 lb-ft).
  3. Reconnect the electrical connector.

• Repeat the above steps for the downstream oxygen sensor (after the catalytic converter).

3. Inspect or Replace Catalytic Converter

• Tools Required: Socket set, torque wrench, exhaust gasket (if needed)
• If the oxygen sensors are functioning correctly, inspect the catalytic converter:
  1. Check for physical damage, blockage, or contamination.
  2. If faulty, remove the catalytic converter by loosening the flange bolts and disconnecting any hangers.
  3. Install the new catalytic converter:
     • Ensure the new converter has the appropriate gaskets.
     • Align the converter and secure it with the flange bolts to the specified torque (usually around 25 lb-ft).

4. Clear Codes and Test Drive

• After the repairs, reconnect the battery.
• Use the OBD-II scanner to clear the P0420 code.
• Perform a test drive to ensure the check engine light does not reappear and the vehicle runs smoothly.