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

Solution

1. Preparation

• Tools Required: OBD-II scanner, socket set, multimeter, torque wrench, exhaust pipe cutter (if necessary), safety glasses, gloves.
• Ensure the vehicle is parked on a level surface, and the engine is cool.
• Disconnect the battery to prevent electrical shorts during repairs.

2. Inspect and Replace Oxygen Sensors

1. Locate the Oxygen Sensors:

   • Identify the upstream (pre-catalytic converter) and downstream (post-catalytic converter) oxygen sensors.

2. Test the Sensors:

   • Use a multimeter to check the voltage output.
   • Upstream sensor should fluctuate between 0.1-0.9 volts while the engine is running.
   • If the downstream sensor is not showing expected values, consider replacing it.

3. Remove and Replace:

   • Use a socket wrench to remove the faulty oxygen sensor.
   • Apply anti-seize compound to the threads of the new sensor.
   • Install the new sensor and torque it to the manufacturer's specifications.

3. Inspect and Replace Catalytic Converter

1. Examine the Catalytic Converter:

   • Look for dents, rust, or signs of overheating.
   • If the structure is compromised, replacement is necessary.

2. Remove the Old Catalytic Converter:

   • Cut exhaust pipes as necessary using an exhaust pipe cutter.
   • Unbolt the catalytic converter from its mounting points.

3. Install New Catalytic Converter:

   • Position the new catalytic converter in place.
   • Secure it with the appropriate bolts and gaskets, tightening them to the manufacturer's torque specifications.

4. Clear Codes and Test Drive

1. Reconnect the Battery:

   • Reconnect the negative battery terminal.

2. Clear the P0420 Code:

   • Use an OBD-II scanner to erase the trouble code from the vehicle's memory.

3. Test Drive:

   • Take the vehicle for a test drive to ensure the check engine light does not reappear and that performance is restored.