how to fix P0420 code in BMW 3 Series F30/F31 (2012-2019)

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery to avoid electrical shorts.
• Allow the engine to cool down before starting work.

2. Inspect and Test Oxygen Sensors

• Tools Required: OBD-II scanner, multimeter, socket set.
• Remove the upstream and downstream O2 sensors using a socket wrench.
• Connect the multimeter to the sensor connectors and check for voltage:
  • Upstream sensor should oscillate between 0.1 to 0.9 volts.
  • Downstream sensor should have a stable voltage around 0.5 volts.
• If either sensor is faulty, replace it.

3. Examine the Catalytic Converter

• Tools Required: Jack and jack stands, socket set.
• Raise the vehicle and secure it on jack stands.
• Inspect the catalytic converter for any physical damage, such as dents or corrosion.
• Tap lightly on the converter; a rattling sound indicates internal damage.
• If damaged, remove the old catalytic converter by loosening the clamps and bolts.
• Replace it with a new catalytic converter, ensuring a proper seal with gaskets.

4. Reassemble and Reconnect

• Reinstall the O2 sensors and ensure the connectors are secure.
• Reconnect the exhaust system, making sure all bolts are tightened to specification (generally around 25-30 Nm).
• Reconnect the battery.