# What does a catalytic converter do? ## Quick answer A catalytic converter is a part in the exhaust that uses precious metals to turn harmful engine gases into less harmful ones. ## What is Catalytic converter? The catalytic converter sits in the exhaust pipe between the engine and the muffler, often close to the engine so it heats up quickly. Inside is a honeycomb coated with metals such as platinum, palladium and rhodium. As hot exhaust passes through, these metals help turn harmful gases into carbon dioxide, water vapor and nitrogen. Most gas cars use a three-way converter, so called because it handles carbon monoxide, unburned fuel and nitrogen oxides at once. To do that, the engine must keep the fuel mixture close to an ideal balance, which is why oxygen sensors and fuel trim matter so much. Larger engines often have two or more converters, one for each bank of cylinders. Converters usually last a long time, often 10 years or more. They tend to fail because of engine problems, such as misfires or burning oil, which overheat or coat the inside. Physical damage from road debris and theft are other common reasons for replacement. On most new US cars, the converter is covered by a federal emissions warranty for 8 years or 80,000 miles. Oxygen sensors before and after the converter let the car's computer check how well it is working. If the converter is not cleaning the exhaust well enough, the computer sets a code such as P0420 or P0430. ## How it works 1. Hot exhaust flows from the engine into the converter. 2. Precious metal coatings on a honeycomb inside speed up chemical reactions. 3. Carbon monoxide, unburned fuel and nitrogen oxides are turned into carbon dioxide, water and nitrogen. 4. An oxygen sensor after the converter lets the computer check that it is still working. ## Types of Catalytic converter | Type | What it is | | --- | --- | | Three-way converter | The standard type on gas cars; it reduces carbon monoxide, unburned hydrocarbons and nitrogen oxides together. | | Close-coupled converter | A converter mounted right at the exhaust manifold so it heats up fast after a cold start, sometimes built into the manifold. | | Diesel oxidation catalyst | Used on diesel engines, usually alongside a diesel particulate filter and often a separate system that reduces nitrogen oxides. | | Upstream and downstream oxygen sensors | The sensors before and after the converter that let the engine computer measure how well it is storing and using oxygen. | ## Common Catalytic converter problems - A P0420 or P0430 code for low converter efficiency on bank 1 or bank 2. - A rattle under the car from a broken internal honeycomb or loose heat shield. - Loss of power, poor acceleration or overheating under the car from a clogged converter. - A rotten egg or sulfur smell from the exhaust. - A sudden loud roar from the exhaust after the converter has been cut out and stolen. ## Frequently asked questions ### What happens when a catalytic converter goes bad? You may see the check engine light with a P0420 or P0430 code, smell rotten eggs or lose power if it is clogged. A rattle under the car can mean the inside has broken up. The car can also fail an emissions test until the converter or its cause is fixed. ### Why are catalytic converters stolen? They contain valuable metals such as platinum, palladium and rhodium. Thieves can cut them out from under some vehicles in minutes, and vehicles with high ground clearance are easier targets. Parking in a garage or lit area and fitting an anti-theft shield can help reduce the risk. ### Can I drive without a catalytic converter? It is illegal to remove a required catalytic converter in the US, and the car will be very loud and fail emissions testing. The check engine light will also stay on. If yours was stolen, the car may still run, but get it repaired before driving far. ### How long does a catalytic converter last? Many catalytic converters last 10 years or more, and often the life of the car. They do not have a replacement interval. When one fails early, an engine problem such as misfires, a rich fuel mixture or burning oil is usually the reason, so that has to be fixed too. ### How much does a catalytic converter replacement cost? At an independent shop, catalytic converter replacement often costs about $1,000 to $3,000, and dealers often charge $1,800 to $4,000. The price depends on the vehicle, the number of converters and whether your state requires a California-certified part. Any engine problem that damaged the old converter adds to the bill. ### Does P0420 always mean a bad catalytic converter? No. P0420 means the computer sees low converter efficiency, but a failing oxygen sensor, an exhaust leak or an engine problem can cause the same code. A good shop checks those first, because replacing the converter without fixing the cause can leave the code coming back. ### What is inside a catalytic converter? Inside the metal shell is a ceramic or metal honeycomb with thousands of tiny channels. The channels are coated with a thin layer containing platinum, palladium and rhodium. The honeycomb gives the exhaust a huge surface area to touch, which helps the chemical reactions happen quickly. ### Can a catalytic converter be repaired? Usually not. If the honeycomb is melted, broken, clogged or coated with oil, the converter is replaced. Loose heat shields and exhaust leaks around it can often be fixed separately. Cleaners and additives rarely restore a converter that has truly failed. ### Do aftermarket catalytic converters work? Many do, if they are the right certified part for your vehicle and state. California and some other states require converters approved for that state. A cheap converter that is not right for the vehicle may not last or may set codes again, so ask about certification and warranty. ### Do diesel engines have catalytic converters? Yes, but they work differently. Most modern diesels use a diesel oxidation catalyst, a diesel particulate filter that traps soot, and often a selective catalytic reduction system that uses diesel exhaust fluid to cut nitrogen oxides. Each part has its own warning lights and maintenance needs. ## Sources - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Related terms - [Oxygen sensor](https://diagxcar.com/glossary/oxygen-sensor) - [OBD-II](https://diagxcar.com/glossary/obd-ii) - [Misfire](https://diagxcar.com/glossary/misfire) - [Check engine light](https://diagxcar.com/glossary/check-engine-light) ## Related pages - [P0420 Catalyst System Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0420) - [P0430 Catalyst System Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0430) - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 - [How long do catalytic converters last?](https://diagxcar.com/how-long/catalytic-converter): 10 years or more --- Source page: https://diagxcar.com/glossary/catalytic-converter Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.