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

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery to prevent electrical issues.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, wrench set, torque wrench.
• Remove the O2 sensor located before the catalytic converter (upstream).
• Inspect the sensor for damage or contamination. Replace if necessary.
• Install the new O2 sensor and tighten to manufacturer specifications (typically around 30 Nm).
• Repeat the process for the downstream O2 sensor.

3. Examine and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, cutting tool (if welding is required), exhaust hanger removal tool.
• Inspect the catalytic converter for physical damage or blockage.
• If replacement is needed, remove the exhaust bolts or clamps connecting the catalytic converter to the exhaust system.
• Remove any exhaust hangers securing the converter.
• Install the new catalytic converter, ensuring proper alignment and fit.
• Torque bolts to manufacturer specifications (usually around 40 Nm).

4. Clear Error Codes

• Reconnect the battery.
• Use an OBD-II scanner to clear the P0420 code from the ECM.
• Perform a test drive to ensure the code does not reappear.