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

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, socket set, jack and jack stands, exhaust backpressure gauge.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Raise the vehicle securely using a jack and jack stands.

2. Inspect Exhaust System

• Sub-steps:
  • Visually inspect the entire exhaust system for cracks, holes, or loose connections.
  • Repair or replace any damaged sections of the exhaust system.

3. Test Oxygen Sensors

• Sub-steps:
  • Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
  • Disconnect the sensors and test their voltage output while the engine is running.
  • For the upstream sensor, voltage should fluctuate between 0.1 to 0.9 volts; for the downstream sensor, it should be stable if the catalytic converter is functioning properly.
  • Replace any faulty oxygen sensors.

4. Inspect and Clean Catalytic Converter

• Sub-steps:
  • If the catalytic converter shows signs of clogging or damage, consider using a catalytic converter cleaning solution.
  • If cleaning does not resolve the issue, proceed to replace the catalytic converter with a new OEM or high-quality aftermarket unit.
  • Ensure all gaskets and seals are replaced when installing a new catalytic converter.

5. Reassemble and Test

• Sub-steps:
  • Reconnect all components and ensure there are no exhaust leaks.
  • Reconnect the negative battery terminal.
  • Start the engine and let it reach operating temperature.