Abstract: The main goal of this paper is to establish whether the Smog-Check system improves local air quality. Using panel data from 2008-2016 for eleven air pollution monitors and a comprehensive dataset of Smog-Checks, I find that an increase in the number of rejections lowers the levels of a small group of pollutants, even after controlling for confounding variables. The main result is that an increase of one standard deviation in the number of rejections decreases [CO], [NO2], [PM10], and [PM2.5] by 5,1%, 6%, 1,4%, and 5,8% of a standard deviation respectively. This is about 16.7% of the effect of what establishing an environmental pre-emergency can accomplish. These results remain significant after changing the empirical specification, and several falsification exercises are conducted to strengthen the identification strategy. Additionally, I explore the potential heterogeneity underlying these results along the station-quality dimension using metrics derived from California’s STAR program, which compares the expected rejection rate versus the realization of this variable, after controlling for a comprehensive set of observable characteristics. I find that rejecting a car by a low-quality station has a negligible impact on air pollution while high-quality stations are the main driver behind the aforementioned results.