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

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the negative battery terminal to ensure safety.
• Raise the vehicle using a jack and secure it with jack stands.

2. Inspect and Replace Oxygen Sensors

1. Locate the O2 Sensors: Find the upstream (pre-catalytic converter) and downstream (post-catalytic converter) oxygen sensors.
2. Remove the Sensors:
   • Use an O2 sensor socket to unscrew the sensors.
   • Disconnect the electrical connectors.
3. Install New Sensors:
   • Apply anti-seize compound to the threads of the new sensors.
   • Install the new sensors and connect the electrical plugs.
   • Torque the sensors to the manufacturer's specification (typically around 30-40 Nm).

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove Exhaust Components:
   • Unbolt the exhaust pipe at the flanges connecting to the catalytic converter.
   • Remove any hangers or brackets securing the exhaust system.
2. Remove the Catalytic Converter:
   • Unfasten the bolts or nuts securing the catalytic converter to the exhaust manifold and the exhaust pipe.
   • Carefully lower and remove the old catalytic converter.
3. Install New Catalytic Converter:
   • Position the new catalytic converter in place.
   • Secure it with new bolts or nuts (check specific torque values).
   • Reattach the exhaust components and ensure all connections are tight.

4. Clear Codes and Reconnect Battery

1. Reconnect Battery: Ensure all tools are removed from the engine bay and reconnect the negative battery terminal.
2. Clear the Error Codes: Use the OBD-II scanner to clear the P0420 code.
3. Test Drive: Drive the vehicle under various conditions to ensure the check engine light does not reappear.