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

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, infrared thermometer, socket set, torque wrench, jack and jack stands.
• Disconnect the negative battery terminal.
• Allow the engine to cool before starting work on the exhaust system.

2. Inspect and Repair Exhaust System

• Sub-steps:
  • Raise the vehicle using a jack and secure it on jack stands.
  • Visually inspect the exhaust system for any cracks, holes, or loose connections.
  • Repair any exhaust leaks found using exhaust sealant or replace damaged sections of the exhaust pipe.

3. Test and Replace Oxygen Sensors (if necessary)

• Sub-steps:
  • Disconnect the electrical connectors from both the upstream and downstream oxygen sensors.
  • Use a multimeter to test the voltage output of each sensor; they should fluctuate between 0.1 to 0.9 volts.
  • If any sensor is faulty, remove it using a socket set and replace with a new one, ensuring proper torque specifications (usually around 30 Nm).
  • Reconnect the electrical connectors securely.

4. Replace Catalytic Converter (if necessary)

• Sub-steps:
  • If the catalytic converter is determined to be faulty based on temperature differential or physical inspection, remove it by loosening the bolts securing it to the exhaust system.
  • Install the new catalytic converter, ensuring proper alignment with the exhaust system and torque bolts to the manufacturer's specifications (typically around 40-50 Nm).

5. Clear Codes and Test

• Sub-steps:
  • Reconnect the negative battery terminal.
  • Use the OBD-II scanner to clear the P0420 code.
  • Start the engine and allow it to reach operating temperature.
  • Perform a test drive to ensure the issue is resolved.