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

Solution

1. Preparation

• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, wrench set, multimeter
• Remove the upstream (pre-catalytic) oxygen sensor:
  1. Locate the upstream O2 sensor near the exhaust manifold.
  2. Use the O2 sensor socket to unscrew the sensor.
  3. Install the new sensor and tighten it to the manufacturer's specifications (typically around 30 Nm).
• Repeat for the downstream (post-catalytic) oxygen sensor if necessary.

3. Inspect and Replace Catalytic Converter

• Tools Required: Jack and jack stands, socket set, exhaust pipe cutter (if needed), torque wrench
• Remove the catalytic converter:
  1. Raise the vehicle securely using jack stands.
  2. Unbolt the exhaust pipe from the catalytic converter using a socket set.
  3. If necessary, cut the exhaust pipe carefully to remove the catalytic converter.
  4. Install the new catalytic converter, ensuring proper alignment.
  5. Tighten all bolts to the specified torque (typically between 30-50 Nm depending on the model).

4. Reconnect Battery and Clear Codes

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 error code from the vehicle's computer.