how to diagnose P0420 code on BMW 5 Series G30/G31 (2017-2024)

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a flat surface, and the engine is cool.
• Disconnect the battery negative terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil.
• Remove the upstream oxygen sensor (pre-catalytic converter) by using the O2 sensor socket.
  • Sub-step: Apply penetrating oil if the sensor is difficult to remove.
• Install the new upstream oxygen sensor and torque it to the manufacturer-specified value (usually around 30 Nm).
• Repeat the process for the downstream oxygen sensor (post-catalytic converter).

3. Inspect and Replace the Catalytic Converter

• Tools Required: Socket set, exhaust pipe cutter, gasket scraper, torque wrench.
• Remove any heat shields or covers obstructing access to the catalytic converter.
• Use the exhaust pipe cutter to cut the exhaust pipes connected to the catalytic converter.
• Remove the catalytic converter from the vehicle.
• Clean the mating surfaces on the exhaust system using a gasket scraper.
• Install the new catalytic converter, ensuring proper alignment and sealing with new gaskets. Torque the bolts to factory specifications.

4. Reconnect Battery and Test

• Reconnect the battery terminal.
• Start the engine and allow it to reach operating temperature.
• Clear the stored trouble codes using the OBD-II scanner.