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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the vehicle battery to prevent electrical shorts.
• Allow the engine to cool before working on the exhaust system.

2. Inspect and Repair Exhaust Leaks

• Sub-steps:
  1. Visually inspect the exhaust pipes and joints for any signs of leakage.
  2. If leaks are found, use a wrench to tighten any loose bolts or clamps.
  3. If damage is present, replace the damaged section of the exhaust using appropriate exhaust pipe connectors.

3. Replace Oxygen Sensors (if faulty)

• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  2. Use a lambda sensor socket to remove the faulty sensors.
  3. Install the new sensors and tighten them to the manufacturer's specified torque (usually around 30 Nm or 22 lb-ft).
  4. Reconnect the electrical connectors.

4. Replace Catalytic Converter (if necessary)

• Sub-steps:
  1. Remove the exhaust components leading to the catalytic converter using a socket set (typically involves removing bolts).
  2. Disconnect the catalytic converter from the exhaust system, ensuring any hangers or brackets are also detached.
  3. Install the new catalytic converter, ensuring proper alignment with the exhaust system.
  4. Secure all components using new gaskets and bolts, tightening to specified torque values.

5. Reconnect Battery

• Reconnect the battery terminals and ensure they are tight.