Common Reasons for Exhaust Emission Testing Failure :
Damaged Emission Control System Component Fault
Damaged or faulty exhaust emission control components are one of the major reasons why vehicles fail a DOT test. the Emission control system is largely responsible for the "breathing" of the engine. in other words, for any car to function efficiently and cleanly, the engine needs to control the correct mixture of fuel to air. If the vehicle emissions are high, then this is usually due to a reduction in the flow of air. This may be due to a simple problem like a blocked air filter, or may indicate a more complicated fault with the vehicles engine managment system.
Engine damage is a common cause of poor exhaust emissions. For example, a blown cylinder head gasket can lead to excessive oil consumption. This in turn will lead to an increase in level of hydro carbons in the exhaust emissions. The same result will be experienced with various other engine component failures such as cylinder rings or damaged pistons.
All modern cars are required to be fitted with a catalytic converter, sometimes referred to a "catalyst" or "cat". The job of the catalytic converter is to break down exhaust gases to less dangerous substances. If the catalytic converter is not working effectively, then it may cause an increase in dangerous emissions to be released from your vehicle.