All cars have one. Lots of cars drive around with it turned on. It tells you to do something that never turns it off. It might turn itself off on the way to the repair shop, or come on as you leave the shop. We could be talking about your back seat driver, but we're not. We're talking about something even more irritating than that. It's called the "check engine" light, known as the amber outline of a little engine glowing on your instrument panel. Why do we have one, and what does it mean?
We have one because U.S. Congress mandated it, under pressure from the environmental lobby. They said that cars had to tell you if they began to pollute the air. That's what the light means, something has happened which has raised the emission level from the car. It might, or might not affect the way the engine runs. It doesn't mean you are low on oil or water, or anything else you can reasonably check yourself, so don't ask me why it says to check your engine, because you can't check for the things that the light is upset about.
There are dozens of sensors all over the car which monitor everything from oxygen in the exhaust to incoming air temperature. They are in turn monitored by a computer system. If one of those sensors detects a reading out of the normal range, it causes the "check engine" light to come on, even if it has little to do with the engine.
For instance, there is a sensor measuring the vapor pressure in your fuel tank, which is controlled so that gas fumes don't escape. This is known as the "evaporative emission system." If the gas cap is left loose, the "check engine" light will come on. You'll do a lot of oil and water checking before you discover that loose cap, and that is only one of many possible places the fumes can leak from. A machine may be required that puts smoke into the tank, to help locate the leak, which can be from many places in a large system of yards and yards of hoses and tubes and valves stretching all over the car.
An evaporative emission leak is only one of many problems that can turn on that one, single light, instead of different lights for each problem. The light does not identify the problem, but the car's computer helps to isolate it. It requires a special scanning tool to interrogate the computer and a trained technician to diagnose the result. There is no such thing as a machine that you can plug in and have it tell you what part to replace, regardless of what you have heard. An x-ray machine doesn't tell us what drugs to take. These are tools to be used by diagnosticians who understand the systems involved, and can suggest remedies for our various ailments and worn out parts.
Some of the problems (like an evaporative emissions leak) may not affect the way the car runs or cause you to break down, which is why some people can ignore the light for years, but some problems might be causing further damage. For instance, if a problem is making the gas mixture too rich, ignoring it could lead to one or more melted catalytic converters in the exhaust, which will have to be replaced when they plug up enough to kill the engine. That can cost big bucks and the rich mixture might have been fixed cheaply. Some people try to bypass some of the emission systems, like removing the catalytic converters, but the computer will sense that they're missing and refuse to control the fuel mixture, keeping the light on forever, and giving terrible gas mileage. Although it is illegal to disable the system, there is no vehicle inspection which would require you to fix an emissions problem in Florida.
If you don't fix it, you should know that when the ice cap melts from global warming and Florida floods over, you caused it, so don't complain about the high water. You saved some money on your car, so use some of it to buy hip boots.
You can't outsmart the computer. You can ignore it at your peril, but the most successful tactic is to find and fix the problems when they occur, to keep the computer happy and our feet dry. If the computer ain't happy, ain't nobody happy, and I, for one, prefer dry feet.