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

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, socket set, torque wrench, exhaust backpressure gauge.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal.

2. Inspect and Replace Oxygen Sensors (if faulty)

1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
2. Remove the electrical connector from the sensors.
3. Use a socket wrench to unscrew the sensors from the exhaust system.
4. Install new oxygen sensors:
   • Apply anti-seize compound to the threads (if recommended).
   • Torque to manufacturer specifications (typically around 30 Nm).
5. Reconnect the electrical connectors.

3. Check and Replace Catalytic Converter (if necessary)

1. Inspect the catalytic converter for signs of damage or blockage.
2. If replacement is needed:
   • Disconnect the exhaust pipes connected to the catalytic converter.
   • Remove any mounting bolts or clamps securing it.
   • Install the new catalytic converter:
     • Ensure proper alignment with exhaust pipes.
     • Torque bolts to manufacturer specifications (typically around 40-60 Nm).
3. Reconnect all exhaust components and ensure there are no leaks.

4. Clear Fault Codes

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