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

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is on a level surface and the engine is cool.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

1. Locate the pre-catalytic and post-catalytic oxygen sensors:
   • Depending on the engine layout, these are typically found on the exhaust manifold and after the catalytic converter.
2. Remove the oxygen sensors:
   • Use an oxygen sensor socket or wrench to unscrew the sensors.
   • Disconnect the electrical connectors gently.
3. Install new oxygen sensors:
   • Apply anti-seize compound on the threads of the new sensors (if required).
   • Screw the new sensors into place and reconnect the electrical connectors.

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove the exhaust components:
   • Use a socket set to unbolt the exhaust flange at the catalytic converter connection.
   • Carefully remove any hangers or brackets holding the exhaust system in place.
2. Inspect the catalytic converter:
   • Check for physical damage, excessive carbon build-up, or signs of clogging.
   • If damage is present, proceed to replace it.
3. Install the new catalytic converter:
   • Position the new catalytic converter in place and secure it with the bolts.
   • Ensure all gaskets are in good condition; replace if necessary.
4. Reattach the exhaust system:
   • Reinstall any hangers or brackets and tighten all connections securely.

4. Clear Codes and Test

1. Reconnect the battery:
   • Reattach the negative battery terminal and ensure it’s tight.
2. Clear the trouble codes:
   • Use the OBD-II scanner to clear the stored P0420 code.
3. Start the vehicle and allow it to reach operating temperature:
   • Monitor the engine performance and check for any reoccurring codes.