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

Solution

1. Preparation

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

2. Inspect and Replace Oxygen Sensors

• Locate the oxygen sensors: The upstream (pre-catalytic) and downstream (post-catalytic) sensors are typically found near the exhaust manifold and after the catalytic converter, respectively.
• Remove the oxygen sensors:
  • Use a specialized oxygen sensor socket to remove the sensors carefully.
  • Disconnect the wiring harnesses from the sensors.
• Install new oxygen sensors:
  • Apply anti-seize compound to the threads of the new sensors.
  • Install the upstream sensor, followed by the downstream sensor, ensuring they are tightened to the manufacturer's specifications (usually around 30 Nm/22 lb-ft).

3. Inspect and Replace Catalytic Converter

• Remove the catalytic converter:
  • Loosen and remove any bolts securing the catalytic converter to the exhaust system using a socket set.
  • Carefully detach the catalytic converter from the exhaust system.
• Install a new catalytic converter:
  • Ensure the new converter is compatible with the G20/G21 model.
  • Position the new catalytic converter in place and tighten the bolts to the manufacturer's specifications (typically 40 Nm/30 lb-ft).

4. Reconnect Battery and Clear Codes

• Reconnect the negative battery terminal.
• Use an OBD-II scanner to clear the P0420 code from the system.