how to fix P0420 code on 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 electrical shorts during repairs.

2. Inspect and Replace Oxygen Sensors

1. Locate the Oxygen Sensors: Identify the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
2. Remove the Upstream Oxygen Sensor:
   • Use a socket wrench and an oxygen sensor socket to carefully unscrew the sensor.
   • Check for damage or contamination on the sensor.
3. Install a New Upstream Oxygen Sensor:
   • Apply anti-seize compound on the threads of the new sensor.
   • Torque the sensor to the manufacturer's specification (typically around 30 Nm).
4. Repeat for Downstream Oxygen Sensor (if necessary):
   • Follow the same steps as above for the downstream sensor.

3. Inspect and Replace the Catalytic Converter

1. Locate the Catalytic Converter: Identify where the catalytic converter is mounted in the exhaust system.
2. Remove Exhaust Components:
   • If applicable, remove any heat shields or brackets securing the catalytic converter.
   • Use a socket set to unbolt the converter from the exhaust manifold and the exhaust pipe.
3. Install the New Catalytic Converter:
   • Position the new catalytic converter in place and secure it with the bolts.
   • Torque the bolts to the manufacturer’s specifications.
4. Reattach Heat Shields: If removed, reinstall any heat shields or brackets.

4. Clear Codes and Test

1. Reconnect the Battery: Reattach the negative battery terminal.
2. Clear the Codes: Use an OBD-II scanner to clear the P0420 code from the system.
3. Test Drive the Vehicle: Drive the vehicle for a short distance to allow the ECU to monitor the new components.