How do I know if my catalytic converter is bad?
Common Symptoms of a Bad Catalytic Converter
A failing catalytic converter often triggers a check engine light with efficiency codes. You might also notice a decrease in fuel economy, sluggish acceleration, or engine misfires.
Physical symptoms include a rattling sound from under the car (due to broken internal honeycomb) and a sulfur-like smell from the exhaust. In severe cases, the engine may stall or fail to start.
- Check engine light with P0420 or P0430
- Rotten egg smell from exhaust
- Rattling noise from under the vehicle
- Reduced engine performance
- Failed emissions test
- Glowing red converter (in extreme cases)
How a Mechanic Diagnoses a Bad Converter
A mechanic may perform a backpressure test by measuring exhaust pressure before the converter; excessive pressure indicates a clog. They might also use an infrared thermometer to check inlet and outlet temperatures; a healthy converter should have a higher outlet temperature.
They will also check oxygen sensor operation and look for other issues that can set efficiency codes. Sometimes a converter is fine but a sensor or exhaust leak is the real problem.
What to Do If You Suspect a Bad Converter
Don't replace the converter based on symptoms alone. Get a proper diagnosis to avoid unnecessary expense.
If replacement is needed, choose a converter that meets your state's emissions requirements. Some states require CARB-compliant converters, which cost more.
Common mistakes
- Assuming a P0420 code always means a bad converter; other issues can cause it.
- Ignoring a bad converter, which can lead to engine damage and failed emissions.
- Buying a used or cheap converter that may not last or be legal in your state.
