It's a scary scenario for most. You're going up a steep mountain trail, your car in its lowest possible safe gear for a mountain road, and the yellow "Service Engine Soon" light blinks on. You coast to a stop, get out and pop the hood and, like most people, wonder if there's something simple like an "off" switch for the light- because you have no idea what to do when the service light comes on in your car.
One thing to remember about this warning signal is that it probably does not mean an immediate emergency; in fact, it may mean no emergency at all. The thing that's scary for most people is the fact that the yellow light is non-specific: it doesn't say "Get Gas" or "Change Oil" or "Engine Tune-up Is Needed." And because the yellow light is non-specific, one has to assume any number of horrific scenarios, all of which may not be horrific at all. Or they may really be bad- it all depends on what's wrong.
So, let's examine what to do when the service light comes on in your car.
Obviously, the yellow "Service Engine Soon" light is a signal from the car's computer system, which originates in the ECM (Electronic Control Module) of the automobile. This is the part of the car that controls both engine and transmission, and while it may be an emergency with either of these, it generally proves to be less than an earthshaking problem.
Usually a yellow light is cautionary, and few of these "Service Engine Soon" lights show themselves in red. So it is NOT an emergency (obviously if the light shows red- and a few do- it's best to get the car serviced at once), but it is something that is keeping your vehicle from running at top efficiency; these eco-friendly days, it may also be something that is preventing your vehicle from achieving its lowest possible emissions level.
Usually, it's quite safe to continue to drive the car; if the problem is minor (as in a lower octane fuel reacting with the fuel pump or the gas cap not properly sealed on the tank) it will correct itself with the next fill-up. The car will automatically correct and turn off the light if this is the case- it's usually just a matter of getting a higher octane fuel and making sure to turn the gas cap securely at the end of the fill-up.
Other reasons for the yellow signal might include a communication failure in the computer system (which can be corrected at most service stations by a diagnostic), or a problem with the vehicle's anti-locking brakes (these cause your brakes to lock in place, and can be corrected by tapping lightly on the brake as you slowly decelerate the vehicle).
Emissions components in the gasoline could be the problem, as the car's engine is attuned to producing the lowest amount of pollution while in operation; there may be trouble with the engine misfiring, or a circumstance that leads to lower fuel mileage (a need for a tune-up or oil change). There may be a potential malfunction of the air bags, or, in the worst case scenario, odors or smoke emanating from the hood. These, best to worst, are some other reasons for the yellow signal.
Obviously the smartest thing is to do exactly as the signal says, and get the car serviced as soon as possible; usually one will find the problem is minor, and the services rendered shouldn't be exorbitant to get that yellow light turned off again.
Those are a few ideas of what to do when the service light comes on in your car.