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

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the negative battery terminal to prevent any electrical issues.
• Allow the vehicle to cool down before beginning work on the exhaust system.

2. Inspect and Replace Oxygen Sensors

1. Remove Upstream Oxygen Sensor

   • Locate the upstream oxygen sensor (before the catalytic converter).
   • Use an oxygen sensor socket to unscrew the sensor carefully.
   • Disconnect the electrical connector from the sensor.

2. Install New Upstream Oxygen Sensor

   • Apply anti-seize compound to the threads of the new sensor.
   • Screw in the new sensor and connect the electrical connector.
   • Torque the sensor to manufacturer's specification (use a torque wrench).

3. Remove Downstream Oxygen Sensor

   • Locate the downstream oxygen sensor (after the catalytic converter).
   • Repeat the removal process as with the upstream sensor.

4. Install New Downstream Oxygen Sensor

   • Apply anti-seize compound to the threads.
   • Install the new sensor and connect the electrical connector.
   • Torque to specification.

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove Exhaust Components

   • If the catalytic converter is suspected to be faulty, remove any exhaust components that obstruct access (use a socket set).
   • Support the exhaust system with jack stands if necessary.

2. Remove Catalytic Converter

   • Unbolt the catalytic converter from the exhaust manifold and the exhaust pipe.
   • Carefully extract the catalytic converter from the vehicle.

3. Install New Catalytic Converter

   • Position the new catalytic converter in place.
   • Bolt it to the exhaust manifold and exhaust pipe, ensuring a proper seal.
   • Torque bolts to the manufacturer's specifications.

4. Reset the Engine Control Module (ECM)

• Reconnect the negative battery terminal.
• Use the OBD-II scanner to clear any existing codes.

5. Test Drive the Vehicle

• Take the vehicle for a test drive to ensure that the check engine light does not reappear and that performance is restored.