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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts.
• Raise the vehicle safely using jack stands if access to the exhaust system is required.

2. Inspect and Replace Oxygen Sensors

• Sub-steps:
  • Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  • Use an O2 sensor socket to remove the sensors.
  • Inspect the sensor wiring for fraying or damage.
  • Install new oxygen sensors, ensuring to apply anti-seize compound on the threads and tightening them to the manufacturer's specifications (typically 30 Nm).

3. Inspect and Replace the Catalytic Converter

• Sub-steps:
  • If the catalytic converter is found to be faulty during inspection, remove the exhaust components leading to the catalytic converter.
  • Use a socket set to unbolt the catalytic converter from the exhaust system.
  • Replace it with a new catalytic converter that meets OEM specifications.
  • Reinstall the exhaust components and tighten bolts to the specified torque (usually around 50 Nm).

4. Clear Codes and Test Drive

• Sub-steps:
  • Reconnect the battery.
  • Use the OBD-II scanner to clear the P0420 code from the ECU.
  • Test drive the vehicle under various conditions (city and highway) to ensure proper operation and no reoccurrence of the code.