Q: Evaporative Emissions System on 2004 Hyundai Sonata

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The check engine light went on in my car. The car ran roughly and almost stalled. There had been a hard rain at the time I had last filled the tank the day before. I have no experience with auto mechanics (or relatively little.) I added to canisters of "dry gas." Initially the car ran well and the check engine light went out. Though the car seemed to run just fine, the check engine light appeared again. I took it to a friend who is a mechanic and he said that he was getting a general code for the Evaporative Emmissions System. I told him I may have driven the car a block or two with the gas cap off. He resent the engine light, told me to fill the tank, and wait a week. Unfortunately, the light came back on. He said that diagnosing and fixing the problem could cost $500 since there are over 30 parts. Is this an accurate estimate?
(1) Answer
You can't really figure an estimate until you perform the test and find out what is causing the evaporative emissions code. You can spend 1-2 hours in labor performing a diagnosis on this system...and then repairs will be needed. $500 is not out of the question; could even be more.

Keep in mind that an evaporative emissions problem will NOT cause a rough running / stalling condition. That is a separate issue. If that happens again, a separate inspection will be necessary.