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

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a flat surface and the engine is cool.
• Disconnect the battery to prevent any electrical shorts.

2. Inspect and Replace Oxygen Sensors

1. Locate the Oxygen Sensors:

   • The upstream sensor is usually located before the catalytic converter, and the downstream sensor is after it.

2. Remove the Oxygen Sensors:

   • Use an oxygen sensor socket to carefully unscrew and remove both the upstream and downstream sensors.
   • Tools Required: Oxygen sensor socket, ratchet, and extensions.

3. Install New Oxygen Sensors:

   • Apply anti-seize compound to the threads of the new sensors (if not pre-applied).
   • Install the new sensors and tighten to the manufacturer's torque specifications (typically around 30 lb-ft).
   • Safety Warning: Avoid touching the sensor tip with your hands to prevent contamination.

3. Check and Replace Catalytic Converter (if necessary)

1. Inspect the Catalytic Converter:

   • Visually inspect for any signs of damage, such as dents or rust.
   • Tap lightly on the converter; if you hear a rattling noise, it may be internally damaged.

2. Remove the Catalytic Converter:

   • Loosen and remove the exhaust clamps and bolts securing the converter.
   • Carefully detach the converter from the exhaust system.
   • Tools Required: Wrench set, socket set, exhaust hanger removal tool.

3. Install the New Catalytic Converter:

   • Position the new catalytic converter in place.
   • Reattach all clamps and bolts to secure the converter, tightening to manufacturer specifications.
   • Safety Warning: Ensure the exhaust system is properly aligned to prevent leaks.

4. Reconnect Battery and Clear Codes

• Reconnect the battery terminals.
• Use an OBD-II scanner to clear the P0420 code from the system.