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

Solution

1. Preparation

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

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil.
  1. Locate the upstream and downstream oxygen sensors (before and after the catalytic converter).
  2. Use penetrating oil to loosen the sensors if they are seized.
  3. Disconnect the electrical connectors from both sensors.
  4. Remove the sensors using an O2 sensor socket.
  5. Install the new sensors by threading them into place and tightening to manufacturer specifications (typically 30 Nm).
  6. Reconnect the electrical connectors.

3. Inspect and Replace Catalytic Converter

• Tools Required: Socket set, torque wrench, exhaust gasket, new catalytic converter.
  1. Raise the vehicle securely on jack stands.
  2. Disconnect the exhaust pipes from the catalytic converter using a socket set.
  3. Remove the catalytic converter from its mounting points.
  4. Inspect and replace any gaskets as necessary.
  5. Install the new catalytic converter and tighten bolts to the specified torque (usually around 40 Nm).
  6. Reconnect exhaust pipes ensuring a secure fit.

4. Final Checks

• Reconnect the negative battery terminal.
• Clear the P0420 code using the OBD-II scanner.