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

Solution

1. Preparation

• Gather tools and parts.
• Disconnect the negative terminal of the battery to ensure safety.

2. Inspect and Replace Oxygen Sensors

1. Locate the oxygen sensors: Identify both the upstream (pre-catalytic converter) and downstream (post-catalytic converter) sensors.
2. Unplug the sensors: Carefully disconnect the electrical connectors.
3. Remove the sensors: Use an oxygen sensor socket to unscrew the sensors from the exhaust pipes.
4. Install new sensors: Apply anti-seize compound on the threads of the new sensors and install them in reverse order of removal, tightening to manufacturer specifications (typically about 30 Nm).
5. Reconnect the sensors: Plug in the electrical connectors securely.

3. Inspect Catalytic Converter

1. Visual Inspection: Check for physical damage or corrosion on the catalytic converter.
2. Check for blockages: If needed, remove the catalytic converter and inspect the honeycomb structure for damage or clogging.
3. Replace if necessary: If the catalytic converter is damaged or clogged, replace it with a new unit as per manufacturer specifications. Ensure proper alignment and secure it with the correct torque (typically around 40 Nm).

4. Clear Codes and Test Drive

1. Reconnect the battery: After repairs, reconnect the negative terminal of the battery.
2. Clear the error codes: Use the OBD-II scanner to erase any stored codes.
3. Test drive the vehicle: Drive the vehicle to allow the ECU to recheck the catalytic converter's efficiency.