Check engine light is on, especially with catalyst efficiency codes like P0420 or P0430
Notice a rotten egg or sulfur smell from the exhaust
Engine feels sluggish or lacks power, especially during acceleration
Fuel economy drops noticeably
Rattling noise comes from inside the converter
Exhaust smells unusually hot or the converter glows red
Vehicle fails an emissions test
Engine misfires or runs rough
Exhaust flow seems restricted
Converter housing is damaged, cracked, or discolored
Backpressure symptoms appear, such as stalling or difficulty revving
Scan tool shows abnormal oxygen sensor readings before and after the converter