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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Park the vehicle on a level surface and engage the parking brake.
• Disconnect the negative battery terminal to prevent any electrical issues.

2. Inspect and Replace Oxygen Sensors

• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors on the exhaust system.
  2. Use an O2 sensor socket to remove the faulty oxygen sensors.
  3. Compare the readings from the old sensors with the specifications for your vehicle.
  4. Install new oxygen sensors and torque them to 30 Nm (22 lb-ft).
• Tools Required: O2 sensor socket, torque wrench.

3. Inspect and Replace the Catalytic Converter

• Sub-steps:
  1. Remove the exhaust pipe connecting the catalytic converter to the rest of the exhaust system.
  2. Unbolt the catalytic converter from its mounting brackets.
  3. Inspect the old catalytic converter for clogs or damage, looking for any signs of overheating.
  4. Install the new catalytic converter, ensuring all gaskets are replaced and torqued to manufacturer specifications, typically around 40 Nm (30 lb-ft).
• Tools Required: Socket set, torque wrench.

4. Reconnect Battery and Clear Codes

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 code and any other related codes.