Question

An emissions test is performed on a six cylinder engine. The test indicates high levels of NOx. The MOST likely cause of this condition is:

A. A rich air fuel mixture.
B. A clogged catalytic converter.
C. A cylinder misfire.
D. A stuck closed EGR valve.

Answer 1

The most likely cause of high levels of Nitrogen Oxides (NOx) in a six-cylinder engine emissions test is a stuck closed Exhaust Gas Recirculation (EGR) valve, because it leads to higher combustion temperatures which facilitate the formation of NOx.

Step-by-step explanation:

An emissions test performed on a six-cylinder engine indicates high levels of Nitrogen Oxides (NOx), which is commonly due to a condition that enables higher combustion temperatures, leading to the formation of these pollutants. The most likely cause of high levels of NOx is D. A stuck closed Exhaust Gas Recirculation (EGR) valve. The EGR valve helps reduce NOx by recirculating a portion of the engine's exhaust gas back to the engine cylinders, thereby lowering combustion temperatures. A stuck closed EGR valve would mean that the exhaust gases are not being recirculated, leading to higher combustion temperatures and, consequently, increased NOx formation. Other causes such as a rich air-fuel mixture, a clogged catalytic converter, or a cylinder misfire typically do not result in elevated NOx levels but may lead to other types of emissions or engine performance issues.