how to diagnose P0420 code in BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather all necessary tools and parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery to avoid electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil
• Sub-steps:
  1. Locate the upstream (pre-catalytic converter) and downstream (post-catalytic converter) oxygen sensors.
  2. Use penetrating oil to loosen the sensors if they are corroded.
  3. Remove the oxygen sensors using the O2 sensor socket.
  4. Install new sensors, tightening them to the manufacturer’s specified torque (typically around 30 Nm).
  5. Reconnect the electrical connectors.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, gasket scraper, new exhaust gaskets, torque wrench
• Sub-steps:
  1. Inspect the catalytic converter for physical damage or blockage.
  2. If replacement is necessary, detach the exhaust pipes connected to the catalytic converter.
  3. Remove the catalytic converter from its mounting points, taking care to collect any loose hardware.
  4. Clean the mounting surfaces to ensure a proper seal with new gaskets.
  5. Install the new catalytic converter and reattach the exhaust pipes, ensuring all bolts are tightened to the specified torque (typically around 25 Nm).
  6. Reconnect the battery.