Emission System

Easy step by step guide on how automotive emissions systems work, though appearances may vary, the process is the same for most vehicles.

Step 1
- The vehicle computer is responsible for the emissions produced by the internal combustion engine it controls. By using many sensors such as an oxygen or mass air flow, which provide feedback data for the computer to analyze and adjust engine emissions, which are controlled to within manufacturer specifications.

Main Computer

Step 2 - The exhaust system oxygen sensor is designed to sense the amount of oxygen in the exhaust system and send feedback data to the computer.

Oxygen Sensor

Step 3 - An EVAP (evaporative emission) solenoid is used to purge raw gas vapor from the fuel tank as to not allow these fumes to enter the atmosphere.

EVAP Purge Solenoid

Step 4 - An electronic throttle actuator is designed to move the throttle plate only when the engine can utilize the fuel being consumed. This control helps unneeded emissions when a driver over advances the throttle pedal.

Electronic Throttle Actuator

Step 5 - A catalytic converter is utilized to expend un-burnt fuel in the exhaust system. The unit is heated by exhaust gases as thermo reaction heats up to achieve the design goal of lowering exhaust emissions. By using primary and secondary oxygen sensors the efficiency of the catalytic converter is monitored. An electric air pump is sometimes used to help heat up the catalytic converters to optimize efficiency.

Catalytic Converter

Step 6 - The mass air flow sensor monitors airflow that enters the engine by measuring resistance across a hot wire that is cooled while various amounts of air pass over it. This feedback data trims the computers adjustments to further help clean emission output.

Mass Air Flow Sensor

Step 7 - The gas cap seals the liquid and fumed gases from escaping the fuel tank. This cap is designed with an automatic tightening mechanism to support a minimum tension the cap must be installed at.

Gas Cap

Step 8 - An EGR valve (exhaust gas recirculation) is use to allow engine exhaust to enter the intake manifold of the engine which cools combustion temperatures and lowers emissions.

EGR Valve

Helpful Information

Emission control systems have been incorporated into automobiles to prevent pollution by limiting the amount of partially burned and evaporated gases into the atmosphere. These systems have been efficient in reducing the air pollution caused by automobiles to a large extent. Increased engine efficiency and lighter vehicle construction has also contributed to helping reduce emissions. The different aspects of emissions are tailpipe emissions, life cycle emissions and evaporative fuel emissions.

Unspent fuel vapors are known as the main contributors to air pollution. These omitted gases consist of carbon monoxide, hydrocarbons, carbon dioxide and nitrogen oxide.

Lifecycle emissions are byproducts which are released into the environment during the manufacturing, maintenance and disposal of the vehicle. These products include volatile solvents such as auto paint and lubricants, manufacturing plant waste, petroleum, heavy metals and many more. The name "catalytic converter" was derived from the operation the device performs. A catalyst material converts partially burned gases to completely burned gases.


Automobiles were first manufactured with exhaust emissions control systems in 1966. Since then, technology has undergone several advancements that have increased the efficiency of the internal combustion engine. Older emission control systems inject air into the exhaust manifold which are known as, air injection reactor (AIR.) The catalytic converter was developed in the early 1970's when severe air pollution was widespread and was made mandatory by the US government in 1976. An EGR valve is used is to lower KNOX (NO2) gases which occur when the combustion chamber when temperature reaches over 3500°.

Tailpipe emission are measured in parts per million or PPM. The first smog emission test was conducted in the mid 1980's. Restrictions in smog laws came later which ushered in the use of a dynamometer so vehicles could be tested as if they were being driven, the number of gases measured also increased.


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.

Article first published