how to fix P0421 code on BMW X5 G05 (2019-2024)

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

1. Locate the oxygen sensors: Identify the upstream (pre-cat) and downstream (post-cat) oxygen sensors in the exhaust system.
2. Remove the old sensors:
   • Use an O2 sensor socket or wrench to carefully unscrew the sensors from their mounts.
   • Disconnect the wiring harness from the sensors.
3. Install new sensors:
   • Apply anti-seize compound to the threads of the new sensors.
   • Screw the new sensors into place and connect the wiring harness.
4. Torque specifications: Tighten the sensors to manufacturer specifications (usually around 30 ft-lbs).

3. Inspect and Replace the Catalytic Converter (if necessary)

1. Check for blockages: Inspect the catalytic converter for signs of clogging or physical damage.
2. Remove the catalytic converter:
   • Loosen the clamps or bolts securing it to the exhaust system.
   • Carefully remove it from the exhaust pipes.
3. Install the new catalytic converter:
   • Position the new catalytic converter and secure it with the clamps or bolts.
   • Ensure all gaskets are in place to prevent leaks.
4. Torque specifications: Tighten the bolts to manufacturer specifications.

4. Reset the Check Engine Light

1. Reconnect the battery: Reattach the negative battery terminal.
2. Clear the fault codes: Use the diagnostic scanner to clear any stored codes from the ECM.

Tools Required:

• OBD-II scanner
• O2 sensor socket or wrench
• Torque wrench
• Exhaust gas analyzer (for testing efficiency)
• Basic hand tools (sockets, wrenches)

Parts Required:

• Upstream oxygen sensor
• Downstream oxygen sensor
• Catalytic converter (if needed)

Estimated Repair Time: 2-4 hours

Safety Warnings:

• Allow the engine to cool before working on the exhaust system.
• Ensure proper ventilation when working under the vehicle.