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

Solution

1. Preparation

• Gather tools: OBD-II scanner, socket set, multimeter, exhaust backpressure gauge, safety gloves, and safety glasses.
• Disconnect the battery to prevent any electrical shorts or shocks during the repair.

2. Oxygen Sensor Replacement (if faulty)

• Tools Required: O2 sensor socket, ratchet, torque wrench
• Remove the upstream and downstream oxygen sensors by:
  1. Locating the sensors on the exhaust manifold (upstream) and catalytic converter (downstream).
  2. Using an O2 sensor socket, carefully unscrew each sensor.
  3. Install the new sensors and torque to manufacturer specifications (usually around 30 Nm).
  4. Reconnect the battery.

3. Catalytic Converter Replacement (if necessary)

• Tools Required: Socket set, torque wrench, exhaust hanger removal tool, safety gloves
• Remove the catalytic converter by:
  1. Lifting the vehicle securely on jack stands.
  2. Removing any exhaust hangers that may obstruct access using an exhaust hanger tool.
  3. Disconnecting the exhaust pipes from the catalytic converter by removing the bolts or clamps.
  4. Carefully removing the catalytic converter from the vehicle.
  5. Installing the new catalytic converter by reversing the removal steps, ensuring all connections are secure.
  6. Torque all bolts to the specified values (consult service manual for exact torque specs).

4. Resetting the Check Engine Light

• After repairs, use the OBD-II scanner to clear the P0420 code and check for any new codes.
• Take the vehicle for a test drive to ensure the check engine light does not reappear.