The evaporative emission control system in a vehicle helps capture fuel emissions that may escape as an engine shuts off and residual fuel sits in fuel lines or valves. Gasoline reverts to a gaseous state when exposed to air and as it dries. These fumes then leak out of the engine into the environment. To avert this problem, federal emission controls require a recapture system with appropriate sensors to signal when the emission capture is not working right. This requirement can then trigger a vehicle's "check engine" light in response.
Diagnosing the Problem
The first sign of trouble with the evaporative emission control system, also known as the EECS, frequently involves a sensor signal being sent to the onboard diagnostic computer and triggering the "check engine" light. However, there are multiple sensors that can cause the trigger, which makes finding the problem harder. Issues can include a loose hose, a bad coal canister where the fumes are captured, a bad signal sensor or a loose gas tank cap. To repair the issue you have to go through a process of elimination.
Start with the Easy Task
When the "check engine" light goes on, plug in a vehicle computer code reader to first confirm the trouble signal. You can buy a basic reader at any automotive parts store for between $50 and $100. This saves the hassle and expense of taking your car to a mechanic to do the same thing. Next, unscrew and reinstall your gas cap. Erase the error code using the car scanner and turn the car on again. By driving your car around you can then determine if the problem will still trigger the "check engine" light or if it was just a loose gas cap. If it was the cap, you saved yourself anywhere from $60 to $150 from an unnecessary mechanic visit.
Other Easy Fixes
The EECS works in a vacuum when it recaptures fumes, so if there is a pressure leak somewhere in the system, it will trigger a problem. This assumes one of the sensors is not malfunctioning. Another common cause can be a loose hose. Using a car repair manual, identify where your hoses are in the engine compartment and check them for tears or cracks. If you find the leak, the fix can be a simple hose exchange. Once finished, again erase the error code from the car and drive it to see if the "check engine" light comes on again.
More Complicated Repairs
The repository for the fumes captured is a coal canister underneath the car. When the engine runs again, the canister is opened and the fumes go back into the engine to burn up. If the coal canister is old, it tends to fail and trigger a minor leak signal. Swapping out the canister with a new one or a replacement from a used parts supplier can solve the leak signal problem. If you know how to work on your car, you can replace it yourself. Otherwise, rely on a mechanic to do the repair.
The Guessing Game
If the issue is not the canister or the gas cap, and the leak is not evident, you will need to have a mechanic perform a diagnostic of the EECS. This involves pressurizing smoke into the system to see where it leaks out. Unless you have the right equipment at home, this repair is best left to a professional mechanic's shop. Unfortunately, finding the problem can require repeat tests, which drive up labor costs on repairs. The same goes for a faulty EECS sensor. Your system may be working fine, but a sensor may have gone bad. A mechanic will need to eliminate the problems one by one.
Ultimately, your car can perform fine without the EECS being repaired, but your "check engine" light will turn on every time you start your car. If you have a code reader, you can confirm the signal and just wipe the code. It can be an annoying task every startup, but it only takes a minute to perform. It is necessary to check, though; otherwise your "check engine" light may be signaling a different problem and you won't know what it is. This option may work for a while until you save up the funds for the professional repair.