how to fix P0420 catalyst efficiency below threshold in BMW 5 Series G30/G31 (2017-2024)

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the battery (negative terminal) to ensure safety.
• Raise the vehicle using a jack and secure it on jack stands.

2. Inspect and Replace O2 Sensors (if necessary)

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) O2 sensors.
  2. Disconnect the electrical connectors from both sensors.
  3. Use the O2 sensor socket to remove the sensors from the exhaust manifold and catalytic converter.
  4. Install new O2 sensors, tightening to manufacturer specifications (typically around 30 Nm).
  5. Reconnect the electrical connectors securely.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, torque wrench, exhaust gasket, penetrating oil.
• Sub-steps:
  1. Remove the exhaust components connected to the catalytic converter, taking care to note their arrangement.
  2. Inspect the catalytic converter for signs of damage or clogging (e.g., rattling noise, visible deformation).
  3. If replacement is needed, remove the old catalytic converter using a socket set.
  4. Install the new catalytic converter with a new exhaust gasket, tightening bolts to the specified torque (usually around 40-50 Nm).
  5. Reattach all exhaust components in the reverse order of removal.

4. Clear Error Codes

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Reconnect the battery.
  2. Use the OBD-II scanner to clear the P0420 code from the engine control unit (ECU).
  3. Perform a test drive to monitor if the code reappears.