how to fix P0420 code in BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the negative battery terminal to prevent electrical issues during repair.
• Allow the exhaust system to cool before working on it.

2. Inspect and Replace Oxygen Sensors

1. Locate the oxygen sensors: Identify the upstream (pre-catalytic converter) and downstream (post-catalytic converter) oxygen sensors.
2. Disconnect the electrical connectors: Carefully unclip the connectors from both sensors.
3. Remove the sensors: Use an oxygen sensor socket to unscrew and remove each sensor.
4. Install new sensors: Apply anti-seize compound to the threads of the new sensors, then install them and tighten to manufacturer's specifications (typically around 30 lb-ft).
5. Reconnect the electrical connectors: Ensure that the connectors are secure.

3. Inspect and Replace Catalytic Converter

1. Remove the exhaust components: Use a socket set to unbolt the exhaust pipe sections connected to the catalytic converter.
2. Inspect the catalytic converter: Check for signs of damage or blockage.
3. Replace the catalytic converter: If damaged, install a new catalytic converter. Ensure you align it properly and use new gaskets if required.
4. Reinstall the exhaust components: Tighten bolts to manufacturer’s specifications and double-check for any exhaust leaks.

4. Clear Codes and Test

1. Reconnect the battery: Reattach the negative battery terminal.
2. Clear the P0420 code: Use the OBD-II scanner to reset the check engine light.
3. Conduct a test drive: Drive the vehicle under different conditions to verify that the check engine light does not reappear.