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

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery to prevent electrical short circuits.
• Allow the engine to cool before beginning work.

2. Inspect and Replace Oxygen Sensors

1. Locate Oxygen Sensors:

   • Identify the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.

2. Remove Oxygen Sensors:

   • Use an O2 sensor socket to carefully unscrew both sensors from their respective locations.
   • Disconnect the electrical connectors.

3. Install New Oxygen Sensors:

   • Apply anti-seize compound to the threads of the new sensors.
   • Install the new sensors by hand and then tighten them with the O2 sensor socket to the manufacturer’s torque specification (usually around 30 ft-lbs).

3. Inspect and Replace Catalytic Converter (if necessary)

1. Inspect the Catalytic Converter:

   • Visually check for any dents, cracks, or discoloration.
   • Use a backpressure gauge to check for restrictions in the exhaust flow.

2. Remove the Catalytic Converter:

   • Unbolt the exhaust flanges connecting the catalytic converter to the exhaust system using a socket set.
   • Remove any additional mounting brackets or hangers.

3. Install New Catalytic Converter:

   • Position the new catalytic converter in place.
   • Bolt it back to the exhaust system, ensuring the gaskets are in good condition or replaced as necessary.
   • Torque the bolts to the manufacturer’s specifications.

4. Reset the ECU

• Reconnect the battery.
• Use the OBD-II scanner to clear the trouble codes from the vehicle's ECU.