I just FIXED this on my god-daughter's car today. It turned out to be a BROKEN HOSE CLAMP LEADING TO THE EVAP CANNISTER. FIXED FOR A TOTAL OF $1.39 IN LESS THAN 30 MINUTES.
Disclaimer - this may not be the issue with your car, but I would highly recommend checking all the hoses and clamps first.
Before you do this, get in your car, close the door, and turn your ignition on and off 3 times (without starting the car) and look at the codes on the digital odometer to read any diagnostic codes that your car is throwing. Write all of the diagnostic codes down for reference.
1) Chock the front tires and jack up the right rear. Make sure that the car is securely jacked and will not move as you need to get under the car!
2) Check all the hoses and clamps for signs of damage or wear; ie: holes or abrasions on the hoses, especially where the hose bends or comes in contact with units.
The hose clamps look to be pretty cheap and after 10 years, it is no wonder that ours broke. Replace any hoses showing wear and replace the plastic bracket with a hose clamp. Any auto parts store will have hose clamps - for less than $2. If the seal is not good to the cannister, then it will throw the code, so it's probably a good idea to replace those anyway since it's so cheap.
3) Tighten the hose clamps.
4) Unchock the car.
5) Disconnect the car battery terminals for a few minutes to clear the codes. You might want to clean the terminals while you're at it - terminal cleaners are about $3 or you can use a (small) splash of your favorite carbonated beverage, ie: Coca Cola products works well. Wipe off ANY excess fluids with a paper towel - make sure the top of the battery is fully dry. You can smear a little dollop of grease on the terminals, a very, very small amount will do. This helps prevent corrosion and prolongs the life of your battery. Never hurts to do this and your battery will last a lot longer and prevents corrosion accumulation.
6) Reattach the terminals and start the car. Leave it running for a minute or two. Turn the car off and switch the ignition on and off 3 times to see if the code is still being thrown.
7) If the code no longer appears, run the car for over 20 miles at least to reset the computer. Check for the code again using the ignition switch trick. If you run it LESS than 20 miles, the computer will NOT reset fully.
Hope this helps!
Source: school of hard knocks