Check engine light is on, especially with codes like P0420 or P0430
Notice a strong sulfur or rotten egg smell from the exhaust
Experience reduced engine power or slow acceleration
Hear rattling noises from underneath the vehicle
Feel the engine struggling at high speeds or under load
See poor fuel economy compared to normal
Notice the vehicle fails an emissions test
Observe excessive heat coming from the converter area
Detect a burning smell near the exhaust
Find the engine misfiring or running rough
See dark exhaust smoke in some cases
Measure unusually high exhaust backpressure
Notice the converter is visibly damaged, melted, or discolored
Experience stalling or difficulty starting after the engine warms up