Check engine light is on
Diagnostic trouble codes related to catalyst efficiency, such as P0420 or P0430
Rotten egg or sulfur smell from the exhaust
Reduced engine performance
Poor acceleration
Decreased fuel economy
Rattling noise from inside the converter
Excessive heat under the vehicle near the converter
Failed emissions test
Dark or discolored exhaust smoke
Engine misfires or rough running
Strong exhaust restriction or loss of power at higher RPMs
Converter housing looks damaged, melted, or cracked
Exhaust backpressure is unusually high