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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery to ensure safety while working on the electrical components.

2. Inspect and Replace Oxygen Sensors (if necessary)

• Tools Required: O2 sensor socket, ratchet, and extension.
• Remove the upstream O2 sensor by locating it before the catalytic converter and using the O2 sensor socket to unscrew it.
• Install the new upstream O2 sensor and torque it to the manufacturer's specifications (typically around 30 Nm).
• Repeat the process for the downstream O2 sensor if it is also faulty.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, torque wrench, and exhaust pipe cutter (if needed).
• Remove the exhaust pipes connected to the catalytic converter by loosening the clamps or bolts.
• Carefully detach the catalytic converter from the exhaust system, noting the orientation for reinstallation.
• Install the new catalytic converter, ensuring it is seated correctly and secure. Torque the bolts to the manufacturer's specifications.
• Reattach the exhaust pipes and clamps.

4. Clear Codes and Test

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