What Three Things a Catalytic Converter Does

A catalytic converter does three core tasks: it reduces nitrogen oxides (NOx) into nitrogen and oxygen, oxidizes carbon monoxide (CO) into carbon dioxide (CO2), and oxidizes unburned hydrocarbons (HC) into carbon dioxide and water. These three reactions—handled by precious-metal catalysts in modern “three-way” converters—dramatically cut smog-forming and toxic emissions from gasoline engines.

The Three Core Functions

The list below outlines the three pollutant-control actions a modern three-way catalytic converter performs in most gasoline vehicles.

• Reduces nitrogen oxides (NOx) to nitrogen (N2) and oxygen (O2) via a reduction catalyst (often rhodium).
• Oxidizes carbon monoxide (CO) to carbon dioxide (CO2) using oxidation catalysts (typically platinum and/or palladium).
• Oxidizes unburned hydrocarbons (HC) to carbon dioxide (CO2) and water (H2O), also via platinum/palladium catalysts.

Together, these reactions target the three primary tailpipe pollutants from gasoline engines—NOx, CO, and HC—helping vehicles meet emissions standards without sacrificing drivability.

How Each Reaction Works

1) Reducing NOx

Inside the converter’s reduction bed, NO and NO2 molecules give up oxygen, which is captured by the catalyst surface and released downstream, leaving mostly harmless nitrogen gas. This step curbs NOx, a major contributor to smog and respiratory irritation.

2) Oxidizing CO

CO, a poisonous gas, is converted to CO2 when it reacts with oxygen on the catalyst surface. While CO2 is a greenhouse gas, this reaction eliminates the acute toxicity of CO and is essential for air-quality goals.

3) Oxidizing Unburned Hydrocarbons

Residual fuel vapors (HC) from incomplete combustion are “burned off” on the catalyst, forming CO2 and water. This significantly reduces smog-forming volatile organic compounds in exhaust.

What Makes It Work: Conditions and Components

Three-way catalysts operate best near the stoichiometric air–fuel ratio (about 14.7:1 for gasoline). Upstream and downstream oxygen sensors help the engine computer maintain this balance, while an oxygen-storage component (often cerium dioxide, or ceria) buffers brief rich/lean swings. The converter must also reach “light-off” temperature—typically 400–600°F (200–315°C)—before it works efficiently, which is why cold-start emissions are higher.

Gasoline vs. Diesel Systems

Diesel engines generally use a different emissions suite: a diesel oxidation catalyst (DOC) to oxidize CO and HC, a particulate filter (DPF) to trap soot, and selective catalytic reduction (SCR) with urea (DEF) to cut NOx. In other words, the three functions in gasoline “three-way” converters are split across multiple components in modern diesels.

Maintenance and Signs of Trouble

The following list highlights common indicators your catalytic converter or related controls may need attention.

• Check Engine Light with codes like P0420/P0430 (catalyst efficiency below threshold).
• Rotten-egg smell (sulfur), misfire history, or poor fuel control leading to overheating or damage.
• Rattling noises (broken substrate) or power loss from clogging.
• Failed emissions test for NOx, CO, or HC despite proper engine tune.

Prompt diagnosis—fixing misfires, O2 sensors, or exhaust leaks—can protect the converter and restore emissions performance.

Summary

A catalytic converter performs three essential tasks: it reduces NOx, and oxidizes CO and unburned hydrocarbons. These reactions, enabled by precious-metal catalysts under tightly controlled air–fuel conditions, are the backbone of emissions control in gasoline vehicles, dramatically cutting smog and toxic pollutants while maintaining engine performance.

Can a car still run without a catalytic converter?

Yes, a car can still run without a catalytic converter, but it will be louder, produce significantly more harmful emissions, and may have reduced fuel efficiency and performance. Driving without a catalytic converter is illegal in many places, will likely cause a safety and emissions inspection failure, and can trigger a “check engine” light due to the computer monitoring the exhaust system. 
Consequences of Driving Without a Catalytic Converter

• Loud Noises: You will notice your car sounds much louder, like a roaring engine, because the catalytic converter helps muffle exhaust noise. 
• Increased Harmful Emissions: The primary function of a catalytic converter is to reduce pollution by converting harmful gases into less harmful ones. Without it, your car will release more pollutants into the air. 
• Poor Performance and Fuel Economy: The engine may not perform as efficiently, potentially leading to a rougher ride, decreased fuel economy, and less smooth acceleration. 
• Check Engine Light: The car’s computer system monitors the catalytic converter’s performance. If it detects the converter is missing or not working correctly, it will turn on the “check engine” light. 
• Inspection Failure: Your vehicle will fail any required safety and emissions inspections, which can prevent you from renewing your vehicle’s registration. 
• Legal Violations: In most places, removing or tampering with a factory-installed catalytic converter is illegal due to federal and state laws regarding emission control devices. 

When It’s Necessary to Avoid Driving 

• If your catalytic converter has been stolen, it’s best to call a tow truck or roadside assistance.
• If you’ve just found your catalytic converter is damaged, it’s recommended to take your car to a trusted repair shop to have it replaced, rather than driving it.

What happens when a catalytic converter goes bad?

When a catalytic converter goes bad, a car may exhibit symptoms like rough idling, reduced engine power and acceleration, a rotten egg smell from the exhaust, a failed emissions test, and a lit check engine light. A bad converter can also lead to decreased fuel economy and, if severely clogged, can even cause the engine to stall or prevent it from starting entirely, making it a serious issue that requires immediate attention.
 
Symptoms of a Bad Catalytic Converter

• Poor engine performance: You might experience a significant loss of power, sluggish acceleration, and difficulty maintaining speed. 
• Rough idling: The engine may misfire or run roughly, leading to vibrations and inconsistent operation. 
• Sulfur or rotten egg smell: A damaged or failing converter can’t properly convert hydrogen sulfide gas, resulting in a sulfurous odor from the exhaust. 
• Illuminated check engine light: Modern vehicles use oxygen sensors to monitor the converter’s efficiency, and a fault will trigger the check engine light. 
• Failed emissions test: A malfunctioning catalytic converter will fail to reduce harmful emissions, leading to a failed smog test. 
• Decreased fuel economy: A restricted exhaust flow can negatively affect the engine’s air-fuel ratio, leading to higher fuel consumption. 
• Rattling noise: If the internal components of the catalytic converter are cracked or broken, you may hear a rattling sound, especially during acceleration. 
• Dark or black smoke: A clogged converter can cause exhaust gases to build up, resulting in darker, denser smoke from the tailpipe. 

Why It Matters

• Environmental impact: A bad catalytic converter allows harmful pollutants to escape into the atmosphere, contributing to air pollution. 
• Engine damage: Ignoring a failing converter can lead to more severe engine problems, such as damaged engine seals or even a complete engine failure. 

What to Do

• Get it diagnosed: Have a mechanic use diagnostic tools to pinpoint the exact issue and determine if the catalytic converter needs replacement. 
• Replace it: A catalytic converter that is damaged or clogged typically needs to be replaced, as cleaning it may not fully restore its function. 

What are the three functions of the catalytic converter?

A three-way catalytic converter has three simultaneous functions:

• Reduction of nitrogen oxides into elemental nitrogen and oxygen: NOx→Nx+Ox.
• Oxidation of carbon monoxide to carbon dioxide: CO+O2→CO2.
• Oxidation of hydrocarbons into carbon dioxide and water: CxH4x+2xO2→xCO2+2xH2O.

What are three things that a catalytic converter does on an automobile?

A catalytic converter is a component of your car’s emissions system that converts toxic pollutants such as hydrocarbons, nitrogen oxides, and carbon monoxide into less toxic emissions such as water vapor, nitrogen, and carbon dioxide.