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

Solution

1. Preparation

• Tools Needed: OBD-II scanner, multimeter, socket set, torque wrench, exhaust back pressure gauge, safety glasses, gloves.
• Safety Warning: Disconnect the battery before working on electrical systems.

2. Inspect and Replace Oxygen Sensors

1. Identify the Sensors:
   • Locate the upstream (pre-catalytic converter) and downstream (post-catalytic converter) oxygen sensors.
2. Test the Sensors:
   • Use a multimeter to check the voltage output of both sensors while the engine is running.
   • The upstream sensor should fluctuate between 0.1 to 0.9 volts, while the downstream should stabilize around 0.5 volts.
3. Replace Faulty Sensors:
   • If faulty, remove the sensor using a socket wrench.
   • Install the new oxygen sensor and tighten to manufacturer specifications (usually around 30 Nm).

3. Inspect and Replace the Catalytic Converter

1. Remove the Exhaust System Components:
   • Loosen and remove any bolts securing the exhaust system to access the catalytic converter.
   • Support the exhaust system to prevent sagging.
2. Remove the Catalytic Converter:
   • Detach the catalytic converter from the exhaust manifold and exhaust pipe.
   • If necessary, cut the old catalytic converter out.
3. Install the New Catalytic Converter:
   • Position the new catalytic converter in place and secure it with bolts to the exhaust manifold and exhaust pipe.
   • Torque the bolts to manufacturer specifications (usually around 40 Nm).

4. Reconnect Battery and Clear Codes

1. Reconnect the Battery:
   • Reattach the negative terminal of the battery.
2. Clear the Codes:
   • Use the OBD-II scanner to clear the P0420 code from the vehicle's memory.
3. Test Drive:
   • Take the vehicle for a test drive to ensure the repair was successful.