Question: how do you know when you need to replace your catalytic converter?
Answer: Decreased Engine Performance. One the easiest-to-notice signs that your catalytic converter needs to be replaced is if your vehicle suddenly and inexplicably begins performing noticeably less well than it has before. ... 
Reduced Fuel Efficiency. ... 
Rattling Noises. ... 
Ignition and Starting Issues. ... 
Consistent Misfires.