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

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Allow the engine to cool if it has been running.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, wrench set, multimeter.
• Remove the upstream (pre-catalytic) oxygen sensor:
  1. Locate the sensor on the exhaust manifold or downpipe.
  2. Use the O2 sensor socket to unscrew the sensor.
  3. Disconnect the electrical connector.
• Install the new sensor:
  1. Apply anti-seize compound to the threads of the new sensor (if required).
  2. Screw in the new sensor by hand, then tighten it with the socket (do not over-tighten).
  3. Reconnect the electrical connector.
• Repeat the process for the downstream (post-catalytic) oxygen sensor if needed.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, jack stands, exhaust pipe cutter (if applicable).
• Remove the catalytic converter:
  1. Support the vehicle securely on jack stands.
  2. Disconnect the exhaust piping from the catalytic converter using a socket set or exhaust pipe cutter.
  3. Remove any mounting bolts or brackets securing the converter.
• Install the new catalytic converter:
  1. Position the new catalytic converter in place, ensuring it aligns with the exhaust system.
  2. Secure it with bolts and brackets, torquing to manufacturer specifications.
  3. Reconnect the exhaust piping and ensure all connections are tight and leak-free.

4. Clear Error Codes

• Using the OBD-II scanner, clear the P0420 code and any other related codes from the engine control module.
• Reconnect the negative battery terminal.