how to fix P0420 code on my BMW 3 Series G20/G21 (2019-2024)

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, torque wrench, multimeter, infrared thermometer, safety glasses, gloves.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors near the catalytic converter.
  2. Disconnect the wiring harness from both sensors carefully.
  3. Use an O2 sensor socket to remove the sensors from their respective positions.
  4. Install the new sensors by reversing the removal process, ensuring to torque them to manufacturer specifications (usually around 30 lb-ft).
  5. Reconnect the wiring harness, ensuring a secure fit.

3. Inspect and Replace the Catalytic Converter

• Sub-steps:
  1. If the catalytic converter is found to be faulty during diagnostic steps, remove it by unbolting the flange connections at both ends using a socket set.
  2. Check for any exhaust leaks or damage to the mounting points.
  3. Install the new catalytic converter, ensuring to align it properly with the exhaust system.
  4. Torque the bolts to the specified manufacturer settings, typically around 25 lb-ft.
  5. Reconnect any sensors or components that were disconnected during the removal.

4. Reset the OBD-II System

• Sub-steps:
  1. Reconnect the negative battery terminal.
  2. Use an OBD-II scanner to clear the P0420 code from the system.
  3. Perform a short test drive to ensure the vehicle operates correctly.