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 by removing the negative terminal.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench, penetrating oil.

• Remove the upstream oxygen sensor:

  1. Locate the upstream O2 sensor near the exhaust manifold.
  2. Apply penetrating oil to the sensor threads if needed.
  3. Use the O2 sensor socket to unscrew and remove the sensor.
  4. Install the new upstream O2 sensor and torque to manufacturer specifications (typically around 30 lb-ft).

• Remove the downstream oxygen sensor:

  1. Locate the downstream O2 sensor after the catalytic converter.
  2. Repeat the process from above to remove and replace it.
  3. Torque the new sensor to manufacturer specifications.

3. Inspect and Replace the Catalytic Converter (if necessary)

• Tools Required: Socket set, torque wrench, exhaust gasket, jack stands.

• Remove the catalytic converter:

  1. Raise the vehicle using jack stands for safety.
  2. Remove the exhaust heat shield if applicable.
  3. Unbolt the catalytic converter from the exhaust manifold and the exhaust pipe.
  4. Remove any old exhaust gaskets.

• Install the new catalytic converter:

  1. Position the new catalytic converter in place.
  2. Use new gaskets and bolts; torque to manufacturer specifications (typically around 25 lb-ft).
  3. Reinstall the exhaust heat shield if applicable.

4. Clear Error Codes and Test

• Reconnect the battery.
• Use the OBD-II scanner to clear the P0420 error code.
• Start the engine and let it warm up to operating temperature.