how to check if my BMW 3 Series F30/F31 (2012-2019) has a bad catalytic converter?

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery to prevent electrical issues during inspection.

2. Scan for Error Codes

• Use an OBD-II scanner to check for any diagnostic trouble codes.
• Document any codes related to the catalytic converter, particularly P0420 (catalytic converter efficiency below threshold).

3. Inspect O2 Sensors

• Locate the upstream and downstream oxygen sensors on the exhaust system.
• Use a multimeter to check the voltage output of both sensors while the engine is running.
• Compare the readings; a significant difference in voltage indicates a potential issue with the catalytic converter.

4. Temperature Check

• After the engine has warmed up, use an infrared thermometer to measure the temperature before (upstream) and after (downstream) the catalytic converter.
• The upstream temperature should be significantly lower than the downstream temperature, with a typical difference of around 100-200°F (37-93°C). If the downstream temperature is lower or equal, the catalytic converter may be failing.

5. Check Exhaust Back Pressure

• Install an exhaust back pressure gauge at the O2 sensor port.
• Start the engine and let it idle, then check the reading. It should not exceed 1.5 psi at idle. Higher pressure indicates a clogged catalytic converter.

6. Smoke Test

• If applicable, perform a smoke test to check for leaks in the exhaust system that could affect the catalytic converter's function.