In the state of Missouri we are required to have our vehicle inspected before we can renew our plates every couple of years. For 1996 and newer vehicle's a safety and emission inspection is required. For a safety inspection the inspector will look at items such as the windshield to make sure it doesn't have any big cracks. Check the wiper blades for smooth operation. Make sure the horn blows. Assure that the brake lights illuminate and that the turn signals blink properly. Suspension parts must be free of play and work as they should. A wheel will also be removed to verify the right amount of brake pad thickness. On the emission side your vehicle will be plugged into with a scanner or on board diagnostic reader (OBD). The reader will check to see if there are any issues and or diagnostic trouble codes (Dtc's) present. The reader will also verify that the check engine light is NOT on. If the light is on it will result in a test failure. If the light is off the reader will check to make sure that all emission related components are operating within specifications. It will make sure the vehicle's oxygen sensors are reading the ideal ratio of fuel and oxygen. That the evaporative emission system is free of leaks. The catalytic converter is doing its job as gasses pass through the exhaust system. While the device is scanning the vehicle it will also check the readiness monitors. The readiness monitors are a series of checkpoints that your vehicle's computer look for every time your vehicle is driven. If the monitors have not cleared since the check engine light was on last will result in a test failure. There are a number of different factors that would trigger that light to come on. One of them is a loose or missing gas cap. A loose cap will cause a small evaporative emission leak and throw a trouble code. If your vehicle's check engine light has ever been on then you may know the hassle it is to get it to go off and stay off. When it is on it could accompany some drivability concerns so it's always best to have a technician you trust take a good thorough look at it.