There are dozens of items checked in a smog check. The main item is a complex examination of all the exhaust gases. The project is to clean up the exhaust emissions generated by millions of gasoline powered vehicles running all over the country. If something is not working correctly, you will have to take your car to a repair station and have it repaired and retested. It is possible to go for a trial smog test where the results are not reported to the DMV. You should never allow your car to be classed as a gross polluter.