The evaporative system (or evap system) in a car is in place to control evaporative emissions. In fact, the vapors that get released from your fuel filler pipe when the gas cap is off are usually much more environmentally harmful than the actual tail pipe gases. This is why the filler neck integrity and the gas cap seal is tested during an emissions test.
Since 1996 the implementation of on-board-diagnostics version 2 (OBDII) required car manufacturers to monitor and control any harmful emissions. Evaporative system problems can cause vapor emissions and that is why when there are problems the check engine light or (malfunction indicator light - MIL or service engine soon - SES) will turn on with a code that can identify what system or what part of the system experienced a fault (not necessarily the cause). So, in other words, tail pipe AND vapor emissions are monitored by the cars OBDII system which use the same indicator light.
Basically, since gas expands in heat, the vapors that sit on top of the tank need a place to go and get stored until they can be used or until the gas cools down and return back to the tank. The system usually consists of numerous valves, sensors, bypass solenoids, pips/hoses, and a container called a carbon canister which is filled with carbon bits (because carbon can store vapor fuel very well). The computer will use these valves, sensors and solenoids to control and monitor what's going on in the system. If there is a problem with this evap system and the light is on, usually the car will feel the same, driveability, and fuel consumption should remain normal. Further damage to the system or even the computer is possible but for the most part unlikely.
To test these systems for leaks we use vacuum pumps or even a smoke machine which can be used to fill the system with smoke and inspect for any escaping smoke. In addition, (if needed) we can hook up to the car's computer and control the systems valves and switches to see sensor or control related problems.
One of the simplest reasons for evap system leak codes is a loose gas cap, gas cap is off or the seal is bad on the gas cap, so the first thing that can be checked is to ensure proper seal at the filler neck/gas cap. Another common mishap is a disconnected hose which can occur on cars that have parts attached to the air filter housing that get detached when an air filter is checked or replaced. Those two are things that can be checked by the owner but beyond that a good technician is most qualified to go any further.
Other interesting facts about the system:
1. It is disabled during the winter and colder weather since gases won't expand. Also note that if this light comes on when spring arrives, the problem may have been there earlier just not monitored until the warmer weather.
2. Usually on most cars the system will only run if the fuel tank level is between 85%-15%.
3. If the gas cap was a problem, the light might not go off right away on it's own until a certain number of key/ignition/drive cycles have happened with no fault detected.
4. Sometimes a computer (car computer) re-flash or update (if available) can correct a code (possibly by adjusting tolerance or repairing a known processing issue).
