how to fix P0420 code on my BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the negative battery terminal to ensure safety during the repair.

2. Inspect and Replace Oxygen Sensors (if faulty)

• Tools Required: Oxygen sensor socket, wrench set, multimeter.
• Remove the upstream oxygen sensor (pre-cat) using the oxygen sensor socket.
  • Disconnect the wiring harness.
  • Install the new oxygen sensor and torque to manufacturer's specifications (typically around 30-35 Nm).
• Remove the downstream oxygen sensor (post-cat) using the oxygen sensor socket.
  • Disconnect the wiring harness.
  • Install the new oxygen sensor and torque to manufacturer's specifications.

3. Check and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, jack stands, wrench set, exhaust gasket.
• Raise the vehicle securely using jack stands.
• Remove the exhaust system components (e.g., exhaust pipe) to access the catalytic converter.
  • Unscrew the bolts holding the catalytic converter in place.
  • Remove the old catalytic converter and replace it with the new one.
  • Ensure to replace the gasket to prevent exhaust leaks. Torque the bolts to manufacturer's specifications.

4. Reassemble Exhaust System

• Reattach any exhaust components that were removed, ensuring all connections are tight and secure.
• Lower the vehicle from jack stands.

5. Reset the Engine Control Module (ECM)

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 code from the ECM.