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

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the negative battery terminal to prevent electrical shock.

2. Inspect and Replace Oxygen Sensors

1. Locate the oxygen sensors: The upstream sensor is before the catalytic converter and the downstream sensor is after it.
2. Remove the oxygen sensors:
   • Use an oxygen sensor socket or wrench.
   • Carefully disconnect the wiring harness from each sensor.
3. Install new oxygen sensors:
   • Apply anti-seize compound on the threads of the new sensors.
   • Tighten to manufacturer specifications (typically 30-40 ft-lbs).
   • Reconnect the wiring harness.

3. Inspect and Replace the Catalytic Converter (if necessary)

1. Inspect the catalytic converter for any signs of damage or excessive carbon buildup.
2. Remove the catalytic converter:
   • Loosen and remove the bolts or clamps securing the converter to the exhaust system.
   • Carefully detach it from the exhaust pipes.
3. Install the new catalytic converter:
   • Position the new converter in place.
   • Secure with bolts or clamps to manufacturer specifications (typically 25-35 ft-lbs).
   • Ensure all seals and gaskets are in good condition and replaced if necessary.

4. Reconnect Battery and Clear Codes

1. Reconnect the negative battery terminal.
2. Use an OBD-II scanner to clear the P0420 code from the ECU.