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

Learn how to diagnose and fix P0420 code related to catalytic converter efficiency 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.
• Raise the vehicle using jack stands for access to the exhaust system.

2. Inspect and Replace Oxygen Sensors

1. Remove the upstream oxygen sensor:
   • Disconnect the electrical connector.
   • Use an O2 sensor socket to unscrew the sensor from the exhaust manifold.
2. Install the new upstream O2 sensor:
   • Apply anti-seize compound to the threads of the new sensor.
   • Install and tighten to manufacturer specifications (typically 30 lb-ft).
   • Reconnect the electrical connector.
3. Repeat for the downstream O2 sensor if faulty:
   • Follow the same steps as above for the downstream sensor.

3. Inspect and Replace Catalytic Converter

1. Remove the catalytic converter:
   • Disconnect the exhaust pipes at the flanges or use a saw if necessary.
   • Unbolt the catalytic converter from the exhaust system.
2. Install the new catalytic converter:
   • Position the new converter and secure it using the appropriate bolts and gaskets.
   • Tighten to manufacturer specifications (typically 30 lb-ft for mounting bolts).
3. Reattach exhaust pipes:
   • Ensure all connections are secure and free of leaks.

4. Clear Codes and Test Drive

1. Reconnect the battery.
2. Use the OBD-II scanner to clear the P0420 code.
3. Test drive the vehicle:
   • Monitor the vehicle for any recurrence of the check engine light or related symptoms.