What does the 3rd catalytic converter do?
In broad terms, the third catalytic converter adds an extra stage of exhaust after-treatment to further reduce pollutants after the first two catalysts. Its exact job varies by vehicle design, but it generally helps clean up remaining emissions and can operate in a different temperature window than the earlier converters.
Understanding catalytic converters and why more than one is used
Modern vehicles use catalytic converters to transform toxic exhaust gases—carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx)—into less harmful substances. Most cars rely on a primary three-way approach, but some designs place a third converter downstream to boost overall emissions performance, especially under varied driving conditions or stricter standards.
Where a third converter fits in
The following points describe common configurations that include a third converter and why manufacturers might install one.
- The third converter is typically positioned downstream of the main catalytic converters, often toward the rear of the exhaust system.
- In many setups, the first converter is close to the engine (a close-coupled or pre-cat), the second is an underbody catalytic converter, and the third is a tertiary converter near the tailpipe for further treatment.
- Diesel engines may use a different downstream aftertreatment sequence (for example, a Diesel Oxidation Catalyst, a Diesel Particulate Filter, and an SCR catalyst). In those cases, the “third converter” role is filled by a downstream system designed for NOx reduction using a reductant like urea (DEF).
- Not all vehicles have a third converter; many rely on two catalysts or integrate aftertreatment in a single unit. The presence and placement depend on the engine, emissions regulations, and packaging constraints.
These configurations aim to maintain high conversion efficiency across engine speeds and driving conditions, ensuring that pollutants are continually reduced from the exhaust stream.
What the third converter does in practice
Specific functions assigned to a downstream or third catalytic converter can include the following, depending on design and application.
- Further oxidize residual carbon monoxide (CO) and unburned hydrocarbons (HC) after the earlier catalysts have done most of the work.
- Provide additional NOx reduction potential, especially when the upstream converters approach their efficiency limits or operate at different temperatures.
- Operate effectively across a broader temperature range, helping emissions stay low during city driving, cold starts, and other changing conditions.
- Complement downstream devices (such as a diesel particulate filter, SCR catalyst, or other aftertreatment) to meet stringent emissions standards and sensor feedback strategies.
In practice, the exact role of a third converter is to extend the cleaning capability of the exhaust system and to ensure compliance with regulatory limits under real-world driving.
Important caveat
Most passenger cars have one or two catalytic converters. A third converter is not universal and is more common in specific designs or diesel applications with additional aftertreatment stages. Always consult the vehicle’s manual or service literature to understand the exact exhaust layout for a given model.
Summary
The third catalytic converter acts as an extra downstream stage in the exhaust system, providing additional pollutant reduction beyond the first two catalysts. Its role varies by design and may include finishing the oxidation of CO and HC, contributing to NOx control, and ensuring emissions performance across different driving conditions. Diesel setups may incorporate notably different downstream aftertreatment (such as SCR) that fulfill a related but separate function. In short, a third converter is about extending the engine’s clean-air performance when regulations or packaging demand it.
What is the purpose of a 3-way catalytic converter?
The three-way catalytic converter currently permits the most efficient, safest and most reliable form of emission control in petrol engines. As the name implies, the three-way catalytic converter converts three pollutants: hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) into less harmful gases.
Why does my car have three catalytic converters?
Most modern cars use three-way catalytic converters. These convert carbon monoxide, unburned hydrocarbons, and nitrous oxides into less harmful gases.
What are the benefits of hollowing out a catalytic converter?
Less Engine Heat
With less pressure throughout the engine and exhaust system, your vehicle's engine is freed up. A large benefit of this is that there's less engine heat. The engine works half as hard without a catalytic converter. Your engine will run more smoothly and be particularly lighter on hot days.
How much scrap is a catalytic converter worth?
A scrap catalytic converter can be worth anywhere from $50 to over $800 or even $1,000, depending on its size, make, model, and the concentration of valuable precious metals like platinum, palladium, and rhodium inside. OEM (original equipment manufacturer) and hybrid converters are often more valuable, while aftermarket converters are worth significantly less, around $5 each. The most accurate price requires getting a quote from a recycler, as the value fluctuates based on precious metal market prices.
Factors that influence the price
- Precious metal content: The primary factor is the amount of platinum, palladium, and rhodium inside. This varies significantly between models.
- Vehicle make and model: Converters from certain vehicles, especially hybrids, are more valuable due to higher precious metal content.
- OEM vs. aftermarket: Original Equipment Manufacturer (OEM) converters are generally much more valuable than aftermarket units, which have significantly less metal.
- Condition: The internal ceramic honeycomb must be intact to be valuable.
- Market fluctuations: The prices of precious metals are constantly changing, which affects the scrap value of the converter.
How to get the most money
- Get a quote: Contact a specialized catalytic converter buyer like RRCats.com for an accurate price estimate before selling.
- Provide details: When getting a quote, be ready to provide photos of the converter, including any serial numbers.
- Sell whole: Do not open the converter yourself, as buyers need the whole unit to assess its value.
- Check local laws: You may need a valid ID or proof of ownership to sell a catalytic converter to a scrapyard.
