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

Solution

1. Preparation

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

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, wrench set, multimeter.
• Remove the upstream (pre-catalytic) oxygen sensor:
  1. Locate the sensor on the exhaust manifold before the catalytic converter.
  2. Use the O2 sensor socket to unscrew the sensor.
  3. Install the new sensor and tighten to manufacturer specifications (typically around 30 Nm).
• Repeat the process for the downstream (post-catalytic) oxygen sensor.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, exhaust pipe cutter, gasket scraper.
• Remove the catalytic converter:
  1. Locate the catalytic converter in the exhaust system.
  2. Use an exhaust pipe cutter to separate the old catalytic converter from the exhaust pipes.
  3. Clean any gasket surfaces to ensure a proper seal with the new converter.
• Install the new catalytic converter:
  1. Position the new converter in place and secure it with bolts.
  2. Ensure all connections are tight and leak-free.
  3. Reconnect any oxygen sensors that were removed.

4. Reconnect Battery and Clear Codes

1. Reconnect the battery terminals.
2. Use the OBD-II scanner to clear the P0420 code from the vehicle’s computer.