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

Solution

1. Preparation

• Gather all tools and parts needed for the repair.
• Disconnect the battery to prevent any electrical short circuits during the process.
• Allow the exhaust system to cool before beginning work.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Remove the upstream oxygen sensor (pre-catalytic converter) by using the O2 sensor socket.
• Check the sensor for contamination or damage.
• If faulty, replace with a new sensor and torque to manufacturer specifications (usually around 30 Nm).
• Repeat for the downstream oxygen sensor (post-catalytic converter) if needed.

3. Inspect and Replace Catalytic Converter (if needed)

• Tools Required: Socket set, jack and jack stands, exhaust pipe cutter (if necessary).
• Raise the vehicle and secure it on jack stands.
• Inspect the catalytic converter for any dents, cracks, or signs of clogging.
• If replacement is necessary, disconnect the exhaust pipes using the socket set.
• Cut any welded sections if required and remove the old catalytic converter.
• Install the new catalytic converter, ensuring proper alignment and securing all bolts to the manufacturer’s torque specifications.

4. Reset the Engine Control Unit (ECU)

• Reconnect the battery.
• Use the OBD-II scanner to clear the P0420 code and reset the ECU.
• Start the engine and let it warm up to normal operating temperature.