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

Solution

1. Preparation

• Gather tools and parts.
• Disconnect the battery to prevent any electrical issues.
• Allow the vehicle to cool down.

2. Inspect and Replace Oxygen Sensors

1. Remove the upstream oxygen sensor (pre-catalytic converter):
   • Locate the sensor in the exhaust manifold.
   • Use an O2 sensor socket to unscrew and remove the sensor.
2. Install the new upstream oxygen sensor:
   • Apply anti-seize compound on the threads of the new sensor.
   • Install the sensor and torque it to 30 Nm (22 lb-ft).
3. Repeat the process for the downstream oxygen sensor (post-catalytic converter) if necessary.

3. Inspect and Replace the Catalytic Converter

1. Remove the catalytic converter:
   • Support the vehicle securely on jack stands.
   • Unbolt the catalytic converter from the exhaust pipes (use a socket set).
   • Remove any heat shields if necessary.
2. Install the new catalytic converter:
   • Position the new converter and align it with the exhaust pipes.
   • Secure it with bolts and torque them to the manufacturer's specifications (typically around 40 Nm or 30 lb-ft).
3. Reattach any heat shields if removed.

4. Final Checks

• Reconnect the battery.
• Clear the trouble codes using the OBD-II scanner.
• Start the engine and check for exhaust leaks.