Exhaust and Catalytic Converter

The exhaust and catalytic converter system is designed to safely move exhaust gasses away from the engine, dampen engine noise, reduce tailpipe emissions and maintain optimum fuel efficiency. These gasses can be harmful to you and the environment if not handled properly. Check to make sure there are no holes in the front section of the exhaust system which may result in poor control of emissions. And make sure exhaust fumes do not enter the passenger compartment where they can cause you serious issues including dizziness, lightheadedness and even death.

The exhaust and catalytic converter system typically contains no moving parts, however, the system is extremely important in the active control of exhaust pollutants. The exhaust system’s manifold and piping carries away the gases created when fuel and air are burned in the engine’s combustion chamber. The Oxygen sensor, an engine management feed-back sensor, located in the front section of the exhaust stream measures how efficiently the fuel and air was burned in the combustion chamber. 

Through precise monitoring of the Oxygen sensor signal the engine management system makes extremely rapid adjustments to the amount of fuel delivered to the combustion chamber maximizing fuel efficiency and creating an exhaust gas mixture optimized for cleanup by the catalytic converter. The exhaust gasses pass through the catalytic converter where the harmful exhaust components: Oxides of Nitrogen, Hydrocarbons and Carbon Monoxides (NOx, HC and CO) are converted to harmless water and Carbon Dioxide (H2O and CO2).

As the converted exhaust gasses leave the catalytic converter they pass over another Oxygen sensor that signals the engine management system how efficiently the catalytic converter was able to clean up the harmful exhaust pollutants. From there the exhaust gasses pass through typical exhaust system components including muffler(s), resonator(s), pipes and tailpipes(s). Let’s take a deeper look at some of the exhaust and catalytic converter components and their functions, including how the catalytic converter changes exhaust gas chemistry.

Exhaust Emissions Overview

Exhaust gasses are made of harmful molecules, but those molecules are made from relatively harmless atoms. Through chemistry and catalyst technology we can split apart the molecules after they leave the vehicle’s combustion into harmless particles before they get pumped out into the air. These processes happen inside a hot catalytic converter.

A catalyst is simply a chemical that makes a chemical reaction go faster without being changed or consumed in the process. In a catalytic converter, the catalyst's job is to speed up the splitting up of the harmful molecules. The catalyst is made from platinum or a similar, platinum-like metal such as palladium or rhodium.

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.

OBD II vehicles 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 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

Exhaust Manifold

The exhaust manifold attaches to the cylinder head and takes exhaust gas from each cylinder and combines it into one pipe. The manifold has traditionally been made of cast iron. Newer manifolds may be constructed from stainless steel, steel or aluminum. For most inline cylinder configurations there is only one exhaust manifold. On engines with V cylinder arrangements, typical of V-6s and V-8s there usually is one exhaust manifold per cylinder bank. Exhaust manifolds operate under extreme conditions with rapid changes in temperature that can cause cracking or gaskets and connection joints to come loose resulting in exhaust gas leaks.

Some exhaust manifolds have the upstream or pre-catalytic converter oxygen sensor threaded into it in a central location that exposes the oxygen sensor tip to a mixture of gasses from all cylinders. If this design is used on a V-6 or V-8 there will be an oxygen sensor in each manifold.

Catalytic Converter

This muffler like part converts harmful carbon monoxide and hydrocarbons to water vapor and carbon dioxide. Some converters also reduce harmful nitrogen oxides. The converter is mounted between the exhaust manifold and the muffler.

The catalytic converter is a sizable cylindrically shaped metal container, positioned in the exhaust stream close to the engine. The converter's inlet pipe is connected to the engine and brings in hot, polluted exhaust gasses from the engine's cylinders. The converter's outlet is connected to the exhaust piping. As the gases from the engine pass through the catalyst, chemical reactions take place on its surface, breaking apart the pollutant gases and converting them into other gases that can be safely returned to the atmosphere.

The temperature at which the catalytic converter begins to work is around 600 degrees F, with the normal operating range around 1,400 degrees F. When unburned fuel is added to the exhaust the converter's operating temperature can go way up. If the temperature gets around 2,000 degrees F or higher the ceramic honeycomb begins to degrade and weaken and the catalyst metals can melt. This accelerates the aging process and causes the converter to lose efficiency. When the converter efficiency has declined to the point where the vehicle may be exceeding the pollution limit the PCM will turn on the Check Engine Lamp and set a diagnostic trouble code.

Unaddressed overheating is the leading cause of catalytic converter plugging. The underlying cause here is often fouled or misfiring spark plugs.

Oxygen Sensor (upstream or Pre–Cat)

All OBD II equipped vehicles use an oxygen sensor to measure how much oxygen is present in the exhaust. The sensor tells the engine management computer (PCM) if the fuel mixture is burning rich (less oxygen) or lean (more oxygen). The PCM continually looks at the sensor’s voltage to determine if the mixture is rich or lean, and adjusts the amount of fuel entering the engine to obtain the correct mixture for maximum fuel economy and low emissions. The oxygen sensor is mounted in the exhaust manifold or close to it in the front exhaust pipe.

The oxygen sensor must be hot (600 degrees F) before it will generate a reliable voltage signal. The hot exhaust gasses provide enough heat to bring an oxygen sensor up to operating temperature during some operating conditions, but not during other conditions such as cold start or idle. During this time the PCM does not use the oxygen sensor signal to adjust the fuel mixture. This typically results in a rich fuel mixture, wasted fuel and higher emissions. Due to these issues OBD II compliant vehicles primarily have heated oxygen sensors.

Heated oxygen sensors have an internal heater circuit that brings the sensor up to operating temperature more quickly than an unheated sensor. The heater will bring the sensor up to operating temperature within 20 to 60 seconds depending on the sensor, and also keep the oxygen sensor hot even when the engine is idling for a long period of time.

When an oxygen sensor signal or heater circuit open circuits, shorts or goes out of range, the PCM usually sets a diagnostic trouble code (DTC) and turns on the Check Engine light. However, oxygen sensors are deemed maintenance items that degrade from usage and should be replaced according to the manufacturer's recommended intervals or when determined to be in a degraded condition. A sensor that is degraded may continue to function well enough not to set a DTC, but not well enough to prevent an increase in emissions and fuel consumption.

The performance of the oxygen sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage or rapid voltage changes. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. It is generally accepted that heated three and four-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles, and the recommended replacement interval for 1996 and newer OBDII-equipped vehicles, is 100,000 miles.

Oxygen Sensor (Downstream or Post–Cat)

On OBD II equipped vehicles, one or two additional oxygen sensors are mounted in or behind the catalytic converter to monitor converter efficiency. There will be one downstream or post-catalytic converter oxygen sensor for each converter if the engine has dual exhausts with separate converters.

The downstream oxygen sensor functions the same as the upstream oxygen sensor in the exhaust manifold. The sensor produces a voltage that changes when the amount of unburned oxygen in the exhaust changes. The high or low voltage signal tells the PCM the fuel mixture is rich or lean.

The PCM monitors converter efficiency by comparing the upstream and downstream oxygen sensor signals. If the converter is doing its job and is reducing the pollutants in the exhaust, the downstream oxygen sensor should show little activity. If the signal from the downstream oxygen sensor starts to mirror that from the upstream oxygen sensor it means converter efficiency has dropped off and the converter isn’t cleaning up the pollutants in the exhaust. When the converter efficiency appears to have declined to the point where the vehicle may be exceeding the pollution limit the PCM will turn on the Check Engine Lamp and set a diagnostic trouble code.


The muffler serves to quiet the exhaust down to acceptable levels. Remember that the combustion process is a series of explosions that create a lot of noise. Most mufflers use baffles to bounce the exhaust around dissipating the energy and quieting the noise. Some mufflers also use fiberglass packing which absorbs the sound energy as the gases flow through. Inside a muffler, you'll find a deceptively simple set of tubes with some holes in them. These tubes and chambers are actually as finely tuned as a musical instrument. They are designed to reflect the sound waves produced by the engine in such a way that they partially cancel themselves out.


The tailpipe is the last piece of exhaust pipe in the system and is vented to open atmosphere. Usually the tailpipe is attached to the output side of the muffler.