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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts.
• Ensure the vehicle is on a level surface and securely lifted if needed.

2. Inspect and Replace Oxygen Sensors

1. Locate the oxygen sensors: Typically found on the exhaust manifold (upstream) and after the catalytic converter (downstream).
2. Disconnect the sensors: Unplug the electrical connectors from the oxygen sensors.
3. Remove the sensors: Use an oxygen sensor socket to unscrew and remove the sensors from the exhaust system.
4. Install new sensors: Apply anti-seize compound to the threads of the new sensors. Install and torque to the manufacturer specifications (usually around 30 lb-ft).
5. Reconnect the sensors: Plug the electrical connectors back into the new oxygen sensors.

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove the exhaust system: If the catalytic converter needs replacement, unbolt the exhaust components that connect to it. Use a socket set and wrench.
2. Remove catalytic converter: Disconnect the catalytic converter from the exhaust pipes by unbolting or cutting if necessary.
3. Install new catalytic converter: Position the new catalytic converter and secure it with bolts. Ensure it aligns with the exhaust system.
4. Torque bolts: Tighten all bolts to the manufacturer specifications, usually around 30-40 lb-ft, but check the specific values for your vehicle.

4. Check for Exhaust Leaks

1. Inspect welds and joints: After reassembly, visually inspect all exhaust joints for leaks.
2. Smoke test: If available, perform a smoke test to identify any leaks in the system.

5. Reconnect Battery and Clear Codes

1. Reconnect the battery: Once repairs are complete, reconnect the battery terminals.
2. Clear error codes: Use the OBD-II scanner to clear the P0420 code and any other codes that were present.