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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to ensure safety during repairs.

2. Replace Faulty Oxygen Sensors

1. Remove the oxygen sensors:
   • Locate the upstream and downstream oxygen sensors.
   • Use an oxygen sensor socket to carefully unscrew and remove both sensors.
2. Install new sensors:
   • Apply anti-seize compound to the threads of the new sensors.
   • Install the new upstream sensor first, followed by the downstream sensor, tightening to manufacturer specifications (usually around 30 Nm).
3. Reconnect the wiring harness to each sensor.

3. Inspect/Replace Catalytic Converter

1. Inspect the catalytic converter:
   • Check for any physical damage or rattling sounds indicating internal failure.
2. Remove the catalytic converter:
   • Unbolt the catalytic converter from the exhaust manifold and the exhaust pipe using a socket set.
   • Carefully lower and remove the unit from the vehicle.
3. Install the new catalytic converter:
   • Position the new catalytic converter and secure it with bolts to the exhaust manifold and exhaust pipe.
   • Ensure all connections are tight and sealed.

4. Clear Codes and Test

1. Reconnect the battery.
2. Clear the P0420 code using the OBD-II scanner.
3. Test drive the vehicle for a minimum of 20-30 minutes to allow the ECM to relearn and monitor parameters.