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

Solution

1. Preparation

• Gather tools and parts needed.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal for safety.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  2. Disconnect the electrical connectors from both oxygen sensors.
  3. Use the O2 sensor socket to remove the upstream oxygen sensor first.
  4. Install the new upstream oxygen sensor and torque to manufacturer's specifications (typically 30 Nm).
  5. Repeat the removal and replacement process for the downstream oxygen sensor.
  6. Reconnect the electrical connectors securely.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, torque wrench, exhaust hanger removal tool.
• Sub-steps:
  1. Inspect the catalytic converter for damage or clogging.
  2. If replacement is necessary, unbolt the catalytic converter from the exhaust system using the socket set.
  3. Remove any exhaust hangers securing the converter using the exhaust hanger removal tool.
  4. Install the new catalytic converter, ensuring it is oriented correctly.
  5. Torque the bolts to the manufacturer's specifications (usually around 40 Nm).
  6. Reattach any exhaust hangers.

4. Reconnect Battery and Clear Codes

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear the P0420 code from the vehicle's computer.
• Start the engine and allow it to reach operating temperature.