Signs of a Bad Catalytic Converter

Signs of a Bad Catalytic Converter

Last Updated September 13, 2023 | Meghan Drummond

The catalytic converter is an important part of your car’s exhaust system. Pretty predictably, there are some easy-to-spot symptoms if you’re having problems with your catalytic converter. Though some vehicles have a cat-less exhaust, vehicles designed to have a catalytic converter require them to be in good working order to function properly.

What a Catalytic Converter Does

The catalytic converter is a honeycomb full of rare minerals that act as a catalyst to help filter harmful contaminants from your exhaust. If your vehicle runs on gasoline or diesel, then there are a variety of gases that are emitted from your exhaust system, including carbon monoxide, nitrogen dioxide, sulfur dioxide, lead, and ground-level ozone.

These gases can cause asthma-like symptoms in people with no prior history of asthma, and they can worsen symptoms in people with preexisting conditions. Nitrogen dioxide, in particular, has been linked to an increased risk of lung cancer, and you can imagine that breathing in lead isn’t actually safer than ingesting it any other way and has roughly the same symptoms.

In short, you want a catalytic converter that works and works well. While an average driver who only commutes for fifteen miles a day and is only ever around their vehicle is unlikely to have dramatic symptoms from their own exhaust, there’s a cumulative effect to consider, and most enthusiasts spend enough time in their car that it can present a problem on its own.

A catalytic converter

Common Signs of a Problem

Check Engine Light

Thankfully, most modern cars have a check engine light that will turn on if any of the sensors discover a problem. One of the more common reasons that a check engine light will appear is if there is a problem with the catalytic converter.

If you see this light, try to check the code and see what’s causing it. If it’s a clogged air or fuel filter, ignoring it could cause your catalytic converter to fail prematurely, which will be significantly more expensive to repair.

Sluggish Acceleration

If you’ve noticed your engine suddenly having a dramatic loss of power and your acceleration isn’t what it was before, then it’s possible you have a clogged catalytic converter. Your engine’s ability to inhale and its ability to exhale are tied together, and a clog in the exhaust system will inevitably create problems with your engine’s ability to mix air and fuel efficiently which will create less powerful combustion and subsequently, less power.

Significantly Worse Gas Mileage

Your engine relies on O2 sensors to adequately mix oxygen and fuel, and if it believes it’s getting more air than it actually is, then it’s going to inject more fuel than you can actually use (and this is true for those with carbureted engines as well). That means you’ll be “running rich” which can cause significant other problems down the line. One way to tell that you’re running rich is to notice the color of the exhaust coming from your vehicle. Vehicles that are burning fuel typically have a thick plume of black exhaust.

The Nose Knows

If you start noticing a particularly foul rotten egg stench while you’re driving, that would be the sulfur that your catalytic converter typically takes care of for you. You may notice other unpleasant smells as well, or even feel a little lightheaded while driving. If it gets to this stage it’s incredibly important that you get your vehicle to a mechanic as soon as possible since many of the exhaust gases expelled by cars can be harmful to your health.

Why do Catalytic Converters Fail?

Most catalytic converters actually will last upwards of ten years, which is pretty great in comparison to many other car parts that require more frequent replacement. Unfortunately, there are things that will cause them to fail prematurely.

Leaded gas is hard to even find in the United States anymore, but leaded gas is absolutely incompatible with a catalytic converter and will cause them to fail in short order. Though some performance vehicles use leaded gas due to its high octane levels, these vehicles aren’t in constant use and don’t typically use a catalytic converter.

Leaks from other parts of your car can also cause your cat to fail before its time. In particular, engine coolant and oil can cause a cat failure. These fluids are what cause a “clogged” catalytic converter.

Though it may sound counterintuitive, a bad spark plug can also cause problems with your catalytic converter. The spark plug’s job is to cause an ignition that will burn the gasoline and air mixture in the combustion chamber. If a plug misfires, or doesn’t spark at all, then that unburned fuel will find its way down into your vehicle and cause problems just about everywhere it goes, including into the cat. Check out this article on how to change your spark plugs if you think they're the problem.

Catalytic Converter Theft

It’s also worth noting that sometimes the problem is actually a total lack of catalytic converter. Because of the rare materials that catalytic converters are made out of (including platinum and palladium), they are a frequent target of thieves. If you notice a very loud sound one morning or see dark exhaust fumes, then you should check under the vehicle to make sure you can still see your catalytic converter.

You can technically drive without your cat, but we’d recommend driving it straight to a shop. Without a catalytic converter, your car is going to be rough riding and remarkably loud. You should also report the theft, both for insurance purposes and also in the hopes that other people won’t have the same rocky start to their morning that you just did.

Going Cat-Less

Some people choose to go cat-less, but there are a few things to consider before making that choice.

The first is that it is illegal. Depending on where you live, you might get hit with a big fine, but you also might have your vehicle impounded.

The horsepower gains that many cat-less advocates talk about are minimal at best. Testing pretty conclusively puts the increase in the single digits. It’s possible it feels like more to them since their brains have been fairly addled by lead poisoning, or they need to exaggerate the horsepower claims in order to justify the smell.

Going cat-less smells. Like rotten eggs and dying brain cells specifically. The person who will be inhaling most of the dangerous chemicals is the driver, of course, but they can also be dangerous to neighbors. If the other person can prove their health problem is due to inhaling unfiltered exhaust and they got your license plate? You’ll likely be staring down a pretty massive lawsuit.

Ultimately, going cat-less is rarely worth it except in special instances, like track vehicles that run on leaded oil or off-roaders. You’ll gain more horsepower by making literally almost any other modification in the world, and it’s hard to get people to ride in your vehicle when it smells like you left an egg salad sandwich under the seat in hundred-degree weather.

Replacing your Catalytic Converter

If it’s time to replace your catalytic converter, then you might want to consider upgrading at the same time with an exhaust system that features high-flow catalytic converters, which will improve your emissions quality while at the same time improving your engine’s power output.

It could be a great time to consider a set of long tube or shorty headers as well so that you can upgrade the entirety of your exhaust system.

This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.