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 parked on a level surface with the parking brake engaged.

2. Inspect and Replace Oxygen Sensors

• Tools Required: OBD-II scanner, socket set, oxygen sensor wrench.
• Sub-steps:
  1. Disconnect the battery (negative terminal) to ensure safety.
  2. Locate the pre-catalytic and post-catalytic oxygen sensors.
  3. Using the oxygen sensor wrench, unscrew the faulty oxygen sensors.
  4. Install the new oxygen sensors, ensuring they are properly tightened (usually to about 30 lb-ft).
  5. Reconnect the battery.

3. Inspect and Replace the Catalytic Converter

• Tools Required: Socket set, torque wrench, exhaust pipe cutter (if necessary).
• Sub-steps:
  1. Lift the vehicle safely using jack stands.
  2. Remove the exhaust heat shield, if equipped, to access the catalytic converter.
  3. Unscrew the bolts holding the catalytic converter in place using the socket set.
  4. Remove the catalytic converter from the exhaust system. If it is welded, use an exhaust pipe cutter.
  5. Install the new catalytic converter, ensuring that the bolts are tightened to the manufacturer's specifications (typically around 30-40 lb-ft).
  6. Reinstall the exhaust heat shield if removed.

4. Reset the OBD-II System

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Use the OBD-II scanner to clear the error codes from the vehicle's computer.
  2. Start the engine and let it run for a few minutes to ensure the check engine light does not reappear.