How Your Vehicle Works

Emission Systems

Emission Systems:

These are the common components of an emissions system:

PCV Valve

The most common emissions device, the PCV (positive crankcase ventilation) valve siphons excess gases that build up in the crankcase and routes them back into the intake manifold, assuring that the gases only flow in that direction. Those gases that build up in the crankcase tend to contain unburned fuel fumes, so the PCV valve plays a big, but simple part in cleaning up emissions.

Catalytic Converter

The purpose of the catalytic converter is to burn unburned hydrocarbons in the exhaust. The catalytic converter is lined with special reactive chemicals that are depleted over time. This means catalytic converters have a limited life.

Oxygen Sensor

Oxygen sensors are perhaps the most important component in a modern vehicle emissions system. They use the oxygen content of the hot exhaust gas to see how cleanly the engine is burning. An improperly operating oxygen sensor can cause the engine computer to run on an abnormally rich fuel/air mixture, which could lead over time to significantly shorter catalytic converter life.

Air Pump

Many vehicles utilize an air pump that adds air at or near the exhaust manifold, or just before the catalytic converter. The added oxygen helps combust unburned gas in the exhaust, and it also aids in the operation of the cat.

EGR Valve

The exhaust gas recirculation valve allows some exhaust gas to be rerouted back into the engine's combustion chambers, to keep combustion cool enough to avoid excess nitrous oxide creation.

Exhaust Seals

If leaky, these can reduce the back pressure in your exhaust system and interfere with the proper operation of the catalytic converter.

Vacuum Hoses

These hoses serve varied purposes, depending on the design of the emissions system, but they generally have a part in keeping (intake) manifold vacuum constant, and in doing so combustion conditions stay at their peak.

Fuel Injectors

Modern fuel injectors are, in a way, emissions devices because they spray and measure fuel for the cleanest, most efficient burn possible. If fuel injector nozzles are slightly clogged and not spraying in the correct pattern, emissions can be affected while otherwise performance might seem normal.


An important component of the emissions system. Although their obvious purpose is to quiet the noise of combustion, mufflers also moderate pressure, assuring that there is enough back pressure for the engine to operate at its intended efficiency and for the catalytic converter to operate at the right temperatures to combust unburned gasoline.

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[Lots more information about mufflers]
[Lots more information about catalytic converters]
[Even more information about catalytic converters]
[Lots more information about CO2 and other greenhouse gasses]


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