how to fix P0420 code on BMW 3 Series G20/G21 (2019-2024)

Solution

1. Preparation

• Gather necessary tools and parts.
• Park the vehicle on a level surface and engage the parking brake.
• Disconnect the negative battery terminal to ensure safety.

2. Inspect and Replace O2 Sensors (if needed)

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Remove the upstream (pre-catalytic converter) oxygen sensor:
  1. Locate the sensor near the exhaust manifold.
  2. Use the O2 sensor socket to unscrew the sensor.
  3. Install the new sensor by hand to avoid cross-threading, then torque to manufacturer specifications (typically around 30 Nm).
• Repeat the process for the downstream (post-catalytic converter) oxygen sensor if it is faulty.

3. Catalytic Converter Inspection and Replacement

• Tools Required: Socket set, torque wrench, exhaust pipe cutter (if necessary), penetrating oil.
• Inspect the catalytic converter:
  1. Check for physical deformities, signs of overheating, or rattling.
  2. If damage is found, proceed to replace the catalytic converter.
• To remove the catalytic converter:
  1. Spray penetrating oil on exhaust bolts to ease removal.
  2. Disconnect any exhaust hangers and the flange connections.
  3. Remove the old catalytic converter, and install the new one by reversing the process.
  4. Torque the bolts to manufacturer specifications (typically around 25 Nm).

4. Reset the Engine Control Module (ECM)

• Reconnect the negative battery terminal.
• Use an OBD-II scanner to clear any codes and reset the ECM.