my BMW 3 Series G20/G21 (2019-2024) has a check engine light and code P0420, what's wrong?

Solution

1. Preparation

• Gather tools and parts listed below.
• Ensure the vehicle is parked on a flat surface and the engine is cool.
• Disconnect the battery to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
2. Disconnect the electrical connectors from both oxygen sensors.
3. Use an oxygen sensor socket to remove the upstream sensor first, followed by the downstream sensor.
4. Install the new oxygen sensors and torque them to the manufacturer's specification (typically around 30-40 Nm).
5. Reconnect the electrical connectors securely.

3. Inspect and Replace Catalytic Converter (if necessary)

1. If the catalytic converter is determined to be faulty, support the exhaust system.
2. Remove any heat shields or brackets securing the catalytic converter.
3. Use a socket set to unbolt the catalytic converter from the exhaust manifold and the downstream exhaust pipe.
4. Install the new catalytic converter, ensuring proper alignment and sealing.
5. Torque the bolts to the manufacturer's specifications.
6. Reattach any heat shields or brackets.

4. Reconnect Battery and Clear Codes

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