surprise, the California Smog Check Test is known to be among the toughest smog tests in the U.S. Although greatly reducing pollution created from cars, the California Smog Check Program did not meet its expectations. Studies have shown that the program managed to only reduce the two main polluting emissions by only 12.3% and 9.8%. (hydrocarbon and carbon monoxide )This is a great letdown since it was predicted to have lowered the polluting rate by 25%.