There are lots of reasons why your vehicle can fail a smog check. Ensure your vehicle performs correctly before arriving at the smog check station. The chances of your vehicle passing a smog inspection are slim to none if your vehicle is not in good working order. If you decide to have a smog inspection completed on a vehicle with known mechanical problems, you will more than likely fail the inspection but will have 90 days to bring your vehicle back for a re-inspection. You can save yourself the trouble and get your vehicle repaired before having it tested. Also, keep in mind that if your check engine light is on, then your vehicle will automatically fail. To investigate the problem, take your vehicle to a repair shop. They will examine your car with a tool that retrieves check engine codes. You can also check with your local auto store, some Pepboys will check your vehicle and give you the engine codes for free.