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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery to ensure safety during work on electrical components.

2. Inspect and Replace O2 Sensors (if necessary)

• Sub-steps:
  • Locate the upstream and downstream oxygen sensors.
  • Use a socket wrench to remove the sensors (typically 22mm).
  • Inspect the sensor connectors for corrosion or damage.
  • Install new sensors by reversing the removal process, ensuring to torque to manufacturer specifications (commonly around 30 Nm).

3. Repair Exhaust Leaks

• Sub-steps:
  • Use a smoke machine or exhaust gas analyzer to identify leaks in the exhaust system.
  • If leaks are found, remove and replace damaged sections of the exhaust or use exhaust sealant for small leaks.
  • Reinstall any removed components and ensure all connections are secure.

4. Replace Catalytic Converter (if necessary)

• Sub-steps:
  • Lift the vehicle securely on jack stands.
  • Remove the bolts securing the catalytic converter to the exhaust system using a socket set.
  • Disconnect any necessary sensor wiring harnesses.
  • Install the new catalytic converter, ensuring it is properly aligned and secured with bolts (torque to specifications).
  • Reconnect any sensors or wiring harnesses that were disconnected.