Function

• The "Service Engine Soon" light, or "Check Engine" light is how a car alerts the driver of any problems related to its central computer. The ECM, or Electronic Control Module, controls the vehicle's engine and transmission. The service engine light warns if there is anything preventing the engine from, "running at top efficiency with the lowest possible emission," according to Samarins.com. In most models, the warning light can be both yellow and red. The yellow light indicates a less urgent problem, while a red light represents a problem that you should address as soon as possible.
  
  

Yellow Warning Lights

• When the light is yellow, it is generally safe to continue driving the car, but you should also be aware of any more signs that it is not performing the best it can. Your car will automatically correct some problems, causing the yellow light to turn off while you are driving. But if the light comes on every time the car is started, it can likely means the problem requires the attention of a mechanic.
  
  

Common Problems

• A yellow light can mean any number of problems with the car's computer, but most likely is indicates an issue with one of a few components. For example, the light could indicate a problem with the performance of the car's anti-lock braking system, which can cause your brakes to lock on when they are used in quick succession. It could indicate a sensor malfunction that may cause your car to display the wrong information on your dashboard, or that there is something preventing your computer from collecting information on a certain part of your engine. The problem could lie with emissions components that ensure your engine is producing the lowest amount of potentially harmful pollutants that it can. A yellow light can also signify a problem with your car's safety restraint systems or air bags.
  
  

Other Symptoms

• There is the possibility that the problem that causes the yellow warning light, is a communication failure that prevents the computer from identifying a bigger problem. When driving with a yellow warning light on, you should have an increased level of awareness for any symptoms of other engine problems. Problems can manifest themselves in a few common ways: difficulty starting the engine, lower than normal fuel mileage, any abnormality in performance, or any odors or visible smoke from under the hood.
  
  

Red Warning Light

• A red light means that there is a problem that is making it potentially unsafe to drive your car, and that you should fix the problem as soon as possible. When a red light turns on while the car is being driven, the driver should pull over and try to make a diagnosis of the problem. To diagnose the problem consult the part of your car's manual that explains the meaning of some warning lights. If the problem can not be fixed without having to drive the car, or a diagnosis can not be reached, the safest thing to do is make arrangements to have the car towed to a mechanic.
  
  

Common Problems - Red

• Like the yellow light, the red light can potentially mean dozens of things. But there are also a few systems that commonly cause the red light to turn on, including a fluid level that is too low, likely anti-freeze, oil, or brake and transmission fluids; an engine belt that is missing or damaged, which can affect your engine's timing and synchronization; or a battery problem like a faulty alternator, or even a completely dead battery.