<human>:  How can I diagnose a bad catalytic converter?
<bot>: The most reliable way to diagnose a bad catalytic converter is to have a certified mechanic run a diagnostic scan of the vehicle's computer system to detect any error codes related to the catalytic converter. Additionally, a visual inspection of the catalytic converter and its connection to the exhaust system may be necessary. Listening for a rattle under the vehicle may also indicate that the catalytic converter has gone bad.