how to fix P0420 code on BMW 3 Series G20/G21 (2019-2024)

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Allow the engine to cool down before starting work on the exhaust system.

2. Inspect and Replace Oxygen Sensors

1. Remove the oxygen sensors:
   • Use an O2 sensor socket to unscrew the upstream and downstream oxygen sensors.
   • Disconnect the electrical connectors carefully.
2. Install new oxygen sensors:
   • Apply anti-seize compound to the threads of the new sensors.
   • Insert and tighten to the manufacturer's specified torque (typically around 30 ft-lbs).
   • Reconnect electrical connectors securely.

3. Inspect and Replace the Catalytic Converter (if necessary)

1. Remove the exhaust system components:
   • Unbolt the exhaust pipe from the catalytic converter using a socket set.
   • If applicable, remove any brackets or hangers holding the catalytic converter in place.
2. Install the new catalytic converter:
   • Position the new catalytic converter and secure it with bolts, ensuring a proper seal.
   • Tighten to the manufacturer's specified torque (usually around 35-45 ft-lbs).
3. Reconnect the exhaust system components:
   • Reattach the exhaust pipe to the new catalytic converter and tighten bolts.

4. Clear Codes and Test

1. Reconnect the battery.
2. Use the OBD-II scanner to clear the P0420 code.
3. Start the engine and allow it to reach operating temperature.
4. Monitor for any check engine light reactivation.