Technical Schmecnical Lesson 3: Catalytic Converters
What is a Catalytic Converter? | How Do Catalytic Converters Work? | Subaru PZEV Vehicles | PZEV Technology

First, a diagram courtesy of Subaru's Drive Magazine Fall 2004 Issue:

Overhead Camshafts - reduced friction from more direct-valve operation, improved efficiency
Electronic Control Units - computer control, greater operating precision
Transmission Gearing - balanced between performance and fuel economy
Four Valves per Cylinder - increased airflow, more complete ignition
Sequential Multi-Port Fuel Injection - precise measurement, reduced fuel waste
Turbocharger (some models) - operated by exhaust gases, greater power without added friction
Catalytic Converter - promote chemical reactions to reduce emissions

In an article from How Stuff Works, Karim Nice and Charles W. Bryant discuss in chemical terms what catalytic converters do and how they work. Nice and Bryant explain:

In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7).

The main emissions of a car engine are:

  • Nitrogen gas (N2) - Air is 78-percent nitrogen gas, and most of this passes right through the car engine.

  • Carbon dioxide (CO2) - This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.

  • Water vapor (H2O) - This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.

These emissions are mostly benign, although carbon dioxide emissions are believed to contribute to global warming. Because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines. Catalytic converters are designed to reduce all three:

  • Carbon monoxide (CO) is a poisonous gas that is colorless and odorless.

  • Hydrocarbons or volatile organic compounds (VOCs) are a major component of smog produced mostly from evaporated, unburned .fuel.

  • Nitrogen oxides (NO and NO2, together called NOx) are a contributor to smog and acid rain, which also causes irritation to human mucus membranes.

How Catalytic Converters Reduce Pollution

In chemistry, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze. In the human body, enzymes are naturally occurring catalysts responsible for many essential biochemical reactions [source: Chemicool].

In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as the materials are extremely expensive. Some of the newest converters have even started to use gold mixed with the more traditional catalysts. Gold is cheaper than the other materials and could increase oxidation­, the chemical reaction that reduces pollutants, by up to 40 percent [source: Kanellos].

Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce.

­The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

2NO => N2 + O2 or 2NO2 => N2 + 2O2

The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2 => 2CO2

There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.

Controlling Pollution and Improving Performance

The third stage of conversion is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

The catalytic converter does a great job at reducing the pollution, but it can still be improved substantially. One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.

One simple solution to this problem is to move the catalytic converter closer to the engine. This means that hotter exhaust gases reach the converter and it heats up faster, but this may also reduce the life of the converter by exposing it to extremely high temperatures. Most carmakers position the converter under the front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm it.

Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people would not wait several minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.

Catalytic converters in diesel engines do not work as well in reducing NOx. One reason is that diesel engines run cooler than standard engines, and the converters work better as they heat up. Some of the leading environmental auto experts have come up with a new system that helps to combat this. They inject a urea solution in the exhaust pipe, before it gets to the converter, to evaporate and mix with the exhaust and create a chemical reaction that will reduce NOx. Urea, also known as carbamide, is an organic compound made of carbon, nitrogen, oxygen and hydrogen. It's found in the urine of mammals and amphibians. Urea reacts with NOx to produce nitrogen and water vapor, disposing more than 90 percent of the nitrogen oxides in exhaust gases [source: Innovations Report].

[Image from Subaru of America, Inc. Drive Magazine]


Subaru produces several models with a PZEV designation, an abbreviation for Partial-Zero Emissions Vehicle, and Subaru's Drive Magazine explains the technology below:


California vehicle standards have led the way in requiring low emissions by vehicles. Set by the California Air Resources Board (CARB), these stringent standards have been adopted by several other states, including Connecticut, Maine, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington. Automakers produce vehicles that meet both these requirements and ones set by the U.S. Environmental Protection Agency (EPA).

CARB defined the PZEV classification. A PZEV vehicle meets Super Ultra Low Emission Vehicle (SULEV) exhaust emission standards and zero evaporative emissions standards. Vehicles meeting these standards are considered the cleanest gasoline-powered vehicles. In addition, the PZEV classification includes a 15-year/150,000-mile (whichever comes first) emission performance and defect warranty, including the On-Board Diagnostic (OBD II) vehicle system, depending on the state in which the vehicle is sold.


At Subaru, protecting the environment is just as important as protecting our passengers. That's why we build fuel-efficient vehicles in ecologically minded plants. Subaru of America, Inc. offers Legacy, Outback, and Forester 2.5L naturally aspirated (non-turbocharged) vehicles that achieve a PZEV rating by the California Air Resources Board. We plan to enhance this product offering in the future. The EPA endorses all Subaru PZEV vehicles as SmartWay®1 certified and promotes them in their Green Vehicle Guide.2


Vehicles with PZEV emissions ratings have such tight pollution controls, and the burning of fuel is so complete, that in very smoggy urban areas, exhaust out of the tailpipe can actually be cleaner than the air outside.

Today's SUBARU BOXER engine has evolved to a level that doesn't require many changes to earn a PZEV label. The differences between non-PZEV and PZEV certified Subaru vehicles can be found in two areas:

  • The catalytic converter

  • The Engine Control Module (ECM) programming

The catalytic converter for a PZEV engine utilizes a higher volume of precious metals than the standard converter. Although that makes the catalytic converter more efficient, it is also more expensive to produce. The PZEV engine's ECM program has been enhanced to operate the engine for reduced emissions.

At a relatively low cost, and without any special equipment, Subaru PZEV vehicles help their owners to contribute to the environment by emitting fewer pollutants to the atmosphere. These vehicles have the same fuel and maintenance requirements as non-PZEV naturally aspirated vehicles, so there's nothing unusual about their care. They do so for a retail price of only $200 more than a non-PZEV Subaru model.

To date, more than 150,000 Subaru owners drive PZEV vehicles.

PZEV Emission Warranty

Although PZEV vehicles are not required by all states, Subaru makes them available throughout the United States. PZEV vehicles are required to be sold in the following states that have adopted the California Air Resources Board (CARB) Low Emission Vehicle (LEV2) standards:

  • California

  • Connecticut

  • Maine

  • Massachusetts

  • New Jersey

  • New York

  • Oregon

  • Pennsylvania

  • Rhode Island

  • Vermont

  • Washington

Only those PZEV vehicles sold and registered in California, Connecticut, Maine, Massachusetts, New Jersey, New York, Rhode Island, and Vermont are eligible for the 15-year/150,000-mile emission warranty.3 (The states of Oregon, Pennsylvania, and Washington elected not to adopt the PZEV emission warranty.)

Catalytic Converter Theft

­All over the country, SUVs and trucks are becoming targets for opportunists looking to cash in on the valuable precious metals used inside catalytic converters. A standard catalytic converter contains several hundred dollars worth of platinum, palladium and rhodium. The ground clearance on trucks and SUVs makes for easy access to the converters, so all a thief needs is a reciprocating saw and about 60 seconds. This trend has police on the lookout in many parts of the country where this kind of theft has been a problem. Police caution SUV and truck drivers to park in busy, well-lit areas, or to park in your garage if you have one, and make sure all doors are locked.

About Technical Schmecnical:

"Technical Schmecnical" is an ongoing series and serves as a helpful guide and resource for vehicle owners to better
understand their vehicles. All of our information comes from owners manuals, quick reference guides, technical manuals,
and even our technicians themselves! Be sure to see our other Technical Schmecnical articles!

Have a great day and Happy Reading!

Subaru Footnotes:
1 SmartWay is a registered trademark of the U.S. Environmental Protection Agency.
2 For 2009 Forester, Legacy, and Outback 2.5L, non-turbo models certified as Partial Zero Emission Vehicles (PZEV) that are sold, registered, and operated in CA, CT, ME, MA, NJ, NY, OR, PA, RI, VT, and WA. PZEV emissions warranty applies only in certain states. See your dealer for complete information on emissions and new car limited warranties.
3 For all Forester, Legacy, and Outback 2.5L non-turbo models certified as PZEV that are sold, registered, and operated only in California and certain other states.


Source:1. Nice, Karim, and Charles W. Bryant.  "How Catalytic Converters Work"  08 November 2000. <>  23 April 2012.

Source 2: Subaru Drive Magazine, Fall 2008 "Its What Makes a Subaru, A Subaru--Controlling Emissions Part Two" <> 23 April 2012.

Source 3: Subaru chassis diagram and labeled components courtesy"Subaru and The Earth" in Subaru Drive Magazine's Fall 2004 Issue <> 23 April 2012.

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