how to troubleshoot P0420 code on BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather tools and parts needed.
• Disconnect the battery to avoid electrical shorts.
• Allow the engine to cool completely before working on the exhaust system.

2. Inspect Oxygen Sensors

• Tools Required: OBD-II scanner, multimeter
• Remove the upstream and downstream oxygen sensors using an O2 sensor socket.
• Test the voltage output of the upstream oxygen sensor; it should fluctuate between 0.1V to 0.9V.
• If the readings are constant or outside this range, replace the sensor.

3. Check for Exhaust Leaks

• Tools Required: Visual inspection tools, smoke machine (optional)
• Inspect exhaust pipes and connections for any cracks or holes.
• If leaks are found, repair or replace the affected components.

4. Inspect and Replace Catalytic Converter (if necessary)

• Parts Required: New catalytic converter (if damaged)
• Remove the exhaust manifold and the catalytic converter from the vehicle using a socket set.
• Install the new catalytic converter, ensuring proper alignment and fit.
• Torque the mounting bolts to the manufacturer's specifications.

5. Reinstall Oxygen Sensors

• Reinstall the upstream and downstream oxygen sensors.
• Torque to specifications (typically around 30 Nm, but check the specific torque values for the model).

6. Reconnect Battery and Clear Codes

• Reconnect the vehicle’s battery.
• Use the OBD-II scanner to clear the P0420 code from the system.