Emissions tests are an annual occurrence and a way to see how much fuel is exiting out of the tailpipe of your vehicle, thus, not being used by your engine. Passing emissions tests means your engine must be properly tuned to utilize the gas entering it and therefore convert it to energy to move the vehicle. Failing an emissions test essentially means you have a gas leak on your hands.
Two categories of failed emissions tests are high HC emissions or high CO emissions. Also, diesel vehicles may experience issues of their own due to high opacity emissions. For non-diesel vehicles, several issues can occur which cause your vehicle’s emissions test to fall into either category. High HC means your engine isn’t converting the fuel into energy and high CO means there is not a good ratio of fuel to air causing incomplete combustion.