Gasoline fuel vehicles cause emissions of airborne pollutants that are poisonous and have a deleterious effect on human health and the environment while natural gas fuels have less harmful effects on the environment and humans due to their greater environmental compatibility. The propose of this study is to investigate the concentration of CO, HC, O₂ in the exhaust outlet of gas and diesel cars that is referred to the technical examination center and to compare the distribution of these gases based on the fuel type of the vehicle and the lambda coefficient. 
Methods: This study was conducted on 1000 vehicles tested in technical examination centers in Shiraz city. To do this, the distribution of air pollutants including CO, H, O₂ exhaust from Pride, Peugeot 405 and Samand, which were obtained with two types of gas and gasoline combustion and the relationship between the distribution of these gases with the lambda coefficient and the gasoline and gas type in these three types of vehicles was analyzed by t-test with a significant level (p <0.05).
Results: The results showed that there is a significant relationship between vehicle fuel type and CO, HC and O₂ emissions from car exhaust (p <0.05) and in gas-fueled cars, it was less gasoline-burned. Also, by increasing the lambda coefficient, the oxygen content of the exhaust was increased while the contaminants, HC, CO decreased. Therefore, with the desired performance of the combustion chamber and the optimum consumption of oxygen in the combustion process, fewer pollutants are produced and introduced into the air.
Conclusion: Also gas-fueled cars have higher lambda rates than gasoline cars and emit less pollution in the air.