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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts.
• Ensure the vehicle is on a level surface and secured on jack stands.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench
• Remove the O2 sensor before the catalytic converter (upstream) using the O2 sensor socket.
  • Warning: Wear safety gloves and goggles.
• Inspect the sensor for damage or contamination.
• If faulty, install a new oxygen sensor and torque it to the manufacturer’s specification (typically around 30 Nm).
• Repeat the process for the O2 sensor after the catalytic converter (downstream).

3. Inspect and Replace the Catalytic Converter

• Tools Required: Socket set, jack, exhaust hanger tool
• Remove the exhaust heat shield if applicable.
• Unbolt the exhaust pipe connected to the catalytic converter.
• Use an exhaust hanger tool to detach the catalytic converter from the exhaust system.
• Inspect for any physical damage or clogging. If damaged, replace it with a new catalytic converter.
• Install the new catalytic converter, ensuring proper alignment and secure all bolts to the recommended torque.

4. Clear Codes and Test Drive

• Reconnect the battery.
• Use the OBD-II scanner to clear the P0420 code from the vehicle’s computer.
• Start the vehicle and allow it to reach operating temperature.
• Take the vehicle for a test drive to verify that the check engine light does not reappear.