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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Ensure the vehicle is parked on a flat surface and the engine is cool.
• Disconnect the negative battery terminal.

2. Inspect and Replace Oxygen Sensors

1. Remove the upstream oxygen sensor:
   • Use an oxygen sensor socket to unscrew the upstream sensor from the exhaust manifold.
   • Disconnect the electrical connector.
2. Install the new upstream oxygen sensor:
   • Apply anti-seize compound to the threads of the new sensor.
   • Screw it into place and tighten to manufacturer specifications (typically around 30 Nm).
   • Reconnect the electrical connector.
3. Repeat for the downstream oxygen sensor if necessary, following the same procedure.

3. Inspect and Replace Catalytic Converter

1. Lift the vehicle safely using jack stands.
2. Remove exhaust components ahead of the catalytic converter:
   • Use a socket set to remove bolts connecting the exhaust pipes.
   • Carefully slide the exhaust components off the catalytic converter.
3. Remove the catalytic converter:
   • Unbolt the catalytic converter from the exhaust manifold and/or exhaust pipe using appropriate tools.
4. Install the new catalytic converter:
   • Position the new converter and secure it with bolts, tightening to specified torque values (typically around 40 Nm).
5. Reattach exhaust components:
   • Reconnect any previously removed exhaust components and tighten all fasteners.

4. Reconnect Battery and Clear Codes

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