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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the vehicle battery.
• Allow the vehicle to cool down completely.

2. Inspect and Replace Oxygen Sensors (if necessary)

• Tools Required: O2 sensor socket, torque wrench
• Remove the upstream (pre-catalytic) oxygen sensor:
  1. Locate the upstream oxygen sensor on the exhaust manifold.
  2. Use the O2 sensor socket to unscrew the sensor.
  3. Install the new O2 sensor by threading it by hand and then tightening it to the manufacturer's specified torque (typically around 30 Nm).
• Repeat the process for the downstream (post-catalytic) oxygen sensor if necessary.

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

• Tools Required: Socket set, jack stands, exhaust pipe cutter, gasket scraper
• Locate the catalytic converter in the exhaust system:
  1. Raise the vehicle using a jack and secure it with jack stands.
  2. Inspect the catalytic converter visually for any signs of damage or blockage.
• If replacement is needed:
  1. Cut the exhaust pipes with an exhaust pipe cutter to remove the old catalytic converter.
  2. Clean the mating surfaces using a gasket scraper.
  3. Install the new catalytic converter, ensuring correct alignment.
  4. Reconnect the exhaust pipes and tighten the clamps to the specified torque.

4. Reset the Engine Control Unit (ECU)

• Tools Required: OBD-II scanner

1. Reconnect the vehicle battery.
2. Use the OBD-II scanner to erase the P0420 code from the ECU.
3. Start the vehicle and allow it to reach operating temperature.