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

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, multimeter, torque wrench, safety goggles, gloves.
• Safety Warnings: Allow the exhaust system to cool before working on it to avoid burns.

2. Replace Oxygen Sensors (if faulty)

• Sub-steps:
  • Locate the upstream and downstream oxygen sensors (usually found before and after the catalytic converter).
  • Disconnect the battery to avoid any electrical shorts.
  • Remove the sensor connectors by pressing the release tab and pulling them apart.
  • Use a socket to unscrew the faulty sensors from their respective locations.
  • Install the new oxygen sensors and torque to manufacturer specifications (usually around 30 lb-ft).
  • Reconnect the battery.

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

• Sub-steps:
  • Remove any heat shields that may be obstructing access to the catalytic converter.
  • Use a socket set to unbolt the catalytic converter from the exhaust system. Take care to support it as you remove it.
  • Inspect the mounting gaskets for wear; replace if necessary.
  • Install the new catalytic converter and torque the bolts to manufacturer specifications.
  • Reattach any heat shields that were removed.

4. Clear Codes and Test Drive

• Sub-steps:
  • Use the OBD-II scanner to clear the P0420 code from the vehicle’s memory.
  • Start the engine and allow it to reach operating temperature.
  • Conduct a test drive to ensure that the system is functioning correctly and monitor for any reoccurrence of the check engine light.