What Does a Catalytic Converter Do?

There was a time when Los Angeles had the outdoor air quality of a rundown dive bar. It looked gross and smelled worse, but the smog generated by cars was also dangerous to breathe. The solution was the catalytic converter. What does a catalytic converter do, exactly, and what happens when yours fails?

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What is a catalytic converter?

A catalytic converter (or cat, for short) is an emissions device that looks like a small muffler connected to the exhaust system. If you were to pull your modern vehicle onto a mechanic’s lift and look up, you’d see the bottom of your vehicle and most likely the entire exhaust system.

From the engine, you would find the exhaust manifold carrying hot spent exhaust gases away from the engine. In modern vehicles, the first catalytic converter might be integrated into the exhaust manifold, followed by another cat later on. An older vehicle might have only one catalytic converter located after the manifold but well before the resonator, muffler and exhaust tips. Inside the catalytic converter are a number of precious metals, such as platinum, palladium and rhodium. They aren’t kidding about the “precious” name, either, as just two ounces of rhodium are worth more than the MSRP of some new cars.

What do catalytic converters do?

A catalytic converter changes dangerous and polluting exhaust gases into less dangerous forms. A “catalyst” is a substance that causes a chemical reaction but is not changed or consumed during that reaction, while a “converter” is a person or thing that converts something. Smooshing those together gets us the explanation that catalytic converters function by an internal substance that chemically changes exhaust gases into something else.

Here’s how that works. Exhaust gases exiting the engine are hot, up to 1,200 degrees Fahrenheit, and contain hydrocarbons, carbon monoxide and nitrogen oxides. Those first two combine to form ozone, what we commonly see as smog. As the hot gases enter the catalytic converter, they encounter a combination of precious metals. These metals are the catalyst, spread out inside the converter on a honeycomb-looking structure. This design allows maximum coverage of the passing exhaust gases with minimum flow restrictions, meaning the catalysts can convert the gases to less toxic forms without slowing down the flow. Testing shows catalytic converters are more than 90% efficient at converting polluting gases into less harmful carbon dioxide, nitrogen and water vapor.

Types of Catalytic Converters

Just as there are many types of vehicle designs, there are a few catalytic converter designs.

Two-way catalytic converters

The two-way catalytic converter has only two tasks: Convert carbon monoxide to carbon dioxide and convert hydrocarbons to carbon dioxide and water. Two-way catalytic converters were an early design, found on 1970s cars. Looking to buy a 1977 Chevy El Camino? It’ll have a two-way converter.

Three-way catalytic converters

Because nitrogen oxide is also an emissions pollutant that needed to be limited, the two-way catalytic converter design was replaced in the early 1980s with the three-way catalytic converter. This improved design has an additional chemical substrate that cleans up the two emissions above and converts dangerous nitrogen oxide to inert nitrogen and oxygen. This is the common catalytic converter design since the introduction of the OBD-II engine diagnostic system.

Diesel catalytic converters

Diesel catalytic converters are technically called diesel oxidation catalysts. They are similar to gasoline catalytic converters as a component in the exhaust stream, converting dangerous gases into less dangerous forms. The diesel oxidation catalyst also focuses on bringing down the levels of particulates, which are sometimes seen as black, sooty exhaust in older or malfunctioning diesels.

What does a catalytic converter do to car performance?

Back in the day of early catalytic converters (and disco), you could often hear about muscle car owners removing the catalytic converter on their 1976 Chevy Camaro Z28 and claiming improved performance. It made sense—anything that impedes airflow will decrease performance to some degree.

However, these were the early two-way catalytic converters placed on engines originally not designed to work with cats. However, on a relatively large and powerful modern engine, like an Aston Martin DB9 with a 5.9-liter V12, removing the catalytic converter is only worth 10 horsepower at 5,600 rpm. In a smaller four-cylinder daily driver, expect to only see an additional one or two horsepower. So, if you’re not making more ponies, what about improving gas mileage?

Unfortunately, the claim that cutting off the catalytic converter will improve gas mileage is a debate that isn’t backed by the science of how cars function. The oxygen sensor, which is located in the exhaust pipe, tries to correct for the weird readings it sees when the cat isn’t present. As a result, your car will increase fuel intake to compensate.

In the end, none of that matters because you are legally required to have a catalytic converter on a street-driven vehicle, even if your state doesn’t have emissions testing or vehicle inspections.

How long does a catalytic converter last?

In many cases, the catalytic converter will last as long as you own the vehicle. While catalytic converter theft is on the rise, a catalytic converter in normal conditions should last around 10 years. Keep this in mind if you regularly buy used cars. The average age of used cars on the road is more than 12 years, so the catalytic converters on these older cars might be suspect or nearing the end of their functional life.

Signs you may need a new catalytic converter

If, after reading all this bad news, you wonder if you need a new catalytic converter, look at the most common symptoms of a failing catalytic converter. If your vehicle runs fine, odds are you are good to go, but here’s what to look out for.

Failed emissions test

Maybe you failed a “sniffer test” with the emissions equipment hooked up to your vehicle, or perhaps the vehicle failed right when the tech connected to the onboard diagnostics. Whichever way your state runs emissions tests, a failure means you need to do some work to fix the vehicle. One of the solutions could be replacing the catalytic converter, because if it’s no longer working, your emissions have massively increased.

Check engine light

If your check engine light is on, you can often have the light diagnosed for free or you can look it up yourself with an inexpensive code reader. One of the failures it might reveal is a broken catalytic converter. When the converter fails, the exhaust flow rate and temperature are out of normal operating limits, sending up that dreaded check engine light.

Sulfur smell

Hydrogen sulfide is a naturally-occurring compound in petroleum products and can cause a big stink in gasoline. When a catalytic converter fails, it is unable to break apart this (or any other) compound. Hydrogen sulfide can be smelled even in low concentrations, which is why a broken catalytic converter is often associated with a rotten egg smell.

Rattling noise from under the vehicle

If the internal (and expensive) catalyst structure breaks apart, the honeycomb structure can sound like random chunks bouncing around inside the converter housing as you drive. The rattling might be more noticeable on startup and will likely get worse over time as the cat breaks down into smaller pieces.

Poor engine performance

Think about how an engine exhaust system works for a moment. The engineers designed the exhaust system so the spent gases could be quickly swept away through a functioning exhaust system. If a catalytic converter fails internally and blocks the rest of the exhaust system with broken pieces, the exhaust flow will be severely reduced from backpressure. This results in sluggish engine performance.

Decreased gas mileage

Say you were averaging 350 miles a tank before, but now you’re struggling to reach 250. This dramatic decrease in gas mileage is common when a catalytic converter fails. Because the exhaust can’t easily clear the blockage, the next intake cycle doesn’t bring in as much fresh air as the engine needs. The result is too much fuel used for not enough movement—and more frequent stops at the pump.