how do I fix the P0420 code on my BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery by removing the negative terminal to prevent electrical shorts.
• Allow the exhaust system to cool before working on it.

2. Replace Faulty Oxygen Sensors

1. Remove Upstream Oxygen Sensor:

   • Use an oxygen sensor socket to unscrew the upstream oxygen sensor from the exhaust manifold.
   • Disconnect the electrical connector.

2. Install New Upstream Oxygen Sensor:

   • Apply anti-seize compound on the threads of the new sensor to prevent future seizure.
   • Screw in the new sensor and connect the electrical connector.

3. Remove Downstream Oxygen Sensor:

   • Repeat the process for the downstream oxygen sensor located after the catalytic converter.

4. Install New Downstream Oxygen Sensor:

   • Follow the same procedure as above for installation.

3. Inspect and Replace Catalytic Converter (if necessary)

1. Remove Exhaust Components:

   • Depending on your model, you may need to remove heat shields or additional exhaust components to access the catalytic converter.
   • Use a socket set to unbolt the catalytic converter from the exhaust system.

2. Install New Catalytic Converter:

   • Place the new catalytic converter in position and secure it using new gaskets and bolts (torque to manufacturer specifications).
   • Reattach any heat shields or additional components removed during disassembly.

4. Reconnect Battery and Clear Codes

• Reconnect the negative battery terminal.
• Use an OBD-II scanner to clear any trouble codes from the system.