Hobbies Cars & Motorcycles Is My Catalytic Converter Bad? Troubleshooting a Catalytic Converter Problem Share PINTEREST Email Print Joe Raedle/Getty Images Cars & Motorcycles Cars How Tos Buying & Selling Basics Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. our editorial process Matthew Wright Updated August 26, 2018 Wondering if you need to replace a bad catalytic converter? Take a look at the following symptoms and see how they compare to your suspected catalytic converter issue. Remember, just because your car is showing one of these symptoms doesn't necessarily mean that your car has gone bad. There are often a number of problems that can cause the same symptom, so it's important to look a little deeper into the issue before you do something drastic—and expensive—like replacing your catalytic converter. How To Tell If the Catalytic Converter is Bad Rotten Egg Smell—If you're regularly getting wafts of rotten egg smell in and around your car—and you've checked to be sure it's not a family member that overindulged at breakfast—you should have your car checked out. Egg smell does not necessarily mean that your catalytic converter has gone bad, but it's often an early sign that can lead to a failed cat. I've seen rotten egg smells in two scenarios usually. The first is a car that is running too rich, meaning the air to fuel ratio is shifted to too much fuel and not enough air. This is a fuel injection or carburetor issue that might be as easy to repair as replacing an oxygen sensor. The second scenario is a vehicle that has already had the cat replaced, but they used a unit that was too small and can't handle the amount of exhaust coming through. Either way, the egg smell means that you are in danger of melting the inside of your converter, which can leave you stranded and means an expensive cat replacement. Rattling Sounds—If you can hear a rattling sound in your catalytic converter while the engine is running, this probably means that your car has begun to break apart. While this is better than melting in that it probably won't leave you stranded on the side of the road, your emission control system won't be doing its job properly, which may mean that you fail your local inspection or emissions test. A rattling catalytic converter will need to be replaced. Glowing Red Hot—A malfunctioning cat can begin to glow bright red after an extended drive. This is not at all normal and means that your converter is very, very hot! A catalytic converter that is partially or fully clogged (also referred to as "plugged") will get so hot that it will begin to glow. An engine that is running too rich can also create excessive heat in the cat, causing it to glow. Replace the cat quickly if you see this symptom! Loss of Engine Power—There are lots of symptoms in a car that can cause you to feel like the engine has lost some of its power. A clogged catalytic converter can certainly cause this, but this symptom requires some additional investigation before you jump to an expensive repair. Check Engine Light—There are lots of problems that can cause a check engine light, but many of them are related to your emission control system, which includes that catalytic converter. While this is often the culprit, things like plug wires, spark plugs, fuel adjustment—and lots more—can be a much less expensive solution. It's always a good idea to go to a major auto parts chain to have your OBD Error Codes read if you have a CEL (Check Engine Light) that continues to come on.