Are you driving around with your light on? All of today's cars have an onboard computer that monitors several systems on your vehicle and gathers information from several sensors such as the engine and then processes this information and controls several engine functions and components, such as fuel mixture, ignition timing, etc. When one of these components or sensors malfunctions, the computer may go into what is called a "limp mode", and will have a limited control of the engine components & performance. When this happens, you may experience poor fuel economy and/or rough idle and poor performance. Also, your check engine light will come on. This is not a call for immediate alarm, so long as all your gauges are in the normal range. Seek service at your first opportunity with us at National Automotive Repair. If left alone for too long it can start to cause other problems or damage.