High HC, or high hydrocarbons, is a term used to describe an increase in the amount of unburned fuel vapors that are detected in a vehicle’s exhaust system. When these vapors are present, it can be caused by a variety of different issues but typically indicates that there is something wrong with either the engine itself or one of its components. High HC levels can cause poor performance and even emissions related problems such as increased smog production. The only way to accurately diagnose and repair any high HC issue is through proper diagnostic testing and troubleshooting which includes examining all relevant systems from intake to exhaust. If any component within this system needs replacing or adjusting then it should be done so immediately so that your vehicle operates properly and doesn’t contribute unnecessary pollution into the environment.