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

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.

2. Inspect and Repair Exhaust Leaks

• Sub-steps:
  • Raise the vehicle using a jack and secure it with jack stands.
  • Inspect the entire exhaust system, focusing on joints and welds.
  • Use exhaust sealant or replace damaged sections as necessary.

Tools Required: Jack, jack stands, exhaust sealant, socket set.

3. Replace Faulty Oxygen Sensors

• Sub-steps:
  • Disconnect the battery to ensure safety.
  • Remove the connector and unbolt the upstream (pre-catalytic) oxygen sensor.
  • Install the new upstream oxygen sensor and reconnect the connector.
  • Repeat for the downstream (post-catalytic) oxygen sensor if required.

Tools Required: O2 sensor socket, wrench set.

4. Replace the Catalytic Converter (if necessary)

• Sub-steps:
  • Disconnect the battery again for safety.
  • Remove the exhaust components connected to the catalytic converter (flanges, clamps).
  • Unbolt and remove the old catalytic converter.
  • Install the new catalytic converter and secure it with the appropriate torque specifications (typically around 30-40 lb-ft).
  • Reconnect the exhaust components and ensure all connections are tight.

Tools Required: Socket set, torque wrench.