how to diagnose P0420 code in a BMW 5 Series G30/G31 (2017-2024)

Safety Warnings

• ⚠Safety Warnings:** Disconnect the battery before working on electrical systems. Allow the exhaust system to cool before inspection.
• ⚠Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
• ⚠Disconnect the sensors' electrical connectors.
• ⚠Use a multimeter to test the voltage output of each sensor while the engine is running:
• ⚠Upstream sensor should fluctuate between 0.1V and 0.9V.
• ⚠Downstream sensor should show a more stable voltage.
• ⚠Replace any faulty oxygen sensors.
• ⚠Use an infrared thermometer to measure the temperature of the exhaust gases before and after the catalytic converter.
• ⚠The inlet temperature should be lower than the outlet temperature by about 100°F to indicate proper operation.
• ⚠If temperatures do not show a significant difference, consider replacing the catalytic converter.
• ⚠Check for any exhaust leaks in the system, particularly around joints and gaskets.
• ⚠Repair or replace any damaged sections of the exhaust system to eliminate leaks that can affect sensor readings.
• ⚠After repairs, reconnect the battery and clear the P0420 code using the OBD-II scanner.
• ⚠Run the vehicle to allow the ECM to recheck the catalytic converter efficiency.
• ⚠Test drive the vehicle under various conditions to ensure the check engine light does not illuminate again.
• ⚠Re-scan for error codes to confirm P0420 has not returned.
• ⚠Monitor the exhaust gases for unusual odors or excessive emissions.
• ⚠Confirm fuel efficiency has returned to normal levels.
• ⚠Regularly inspect and maintain the exhaust system.
• ⚠Ensure the air-fuel mixture is correctly adjusted and that no fuel injectors are leaking.
• ⚠Replace oxygen sensors as part of routine maintenance according to manufacturer recommendations.
• ⚠Address any engine performance issues promptly to prevent damage to the catalytic converter.