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

Solution

1. Preparation

• Gather tools and parts.
• Ensure the vehicle is parked on a flat surface and the engine is cool.
• Disconnect the battery to prevent any electrical shorts.

2. Inspect and Replace Oxygen Sensors

1. Remove the upstream (pre-catalytic) oxygen sensor:

   • Use an O2 sensor socket to unscrew the sensor from the exhaust manifold.
   • Ensure you have a replacement sensor ready.

2. Install the new upstream oxygen sensor:

   • Apply anti-seize compound (if recommended) to the threads of the new sensor.
   • Torque the sensor to the manufacturer’s specification (typically around 30 Nm).

3. Remove the downstream (post-catalytic) oxygen sensor:

   • Repeat the same steps as above for the downstream sensor.

4. Install the new downstream oxygen sensor:

   • Use the same procedure as for the upstream sensor.

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

1. Disconnect the exhaust system:

   • Loosen and remove the bolts securing the catalytic converter to the exhaust pipe and manifold.

2. Remove the catalytic converter:

   • Carefully extract the catalytic converter from the mounting points.

3. Install the new catalytic converter:

   • Position the new catalytic converter in place and secure it using new gaskets and bolts.
   • Torque to specified values (consult service manual for exact numbers).

4. Reconnect the Battery

• Reconnect the vehicle battery and ensure all connections are tight.

5. Clear Codes and Test

• Use the OBD-II scanner to clear the P0420 code.
• Start the engine and allow it to reach operating temperature.