1999 Toyota Corolla CHECK ENGINE light, canister valve?

I have a 1999 Toyota Corolla with about 75K miles on it. Several weeks ago, the CHECK ENGINE light went on. I took it into a shop immediately. They said that the "canister valve" went bad and it'd cost $400 to replace it. Said it was a common problem with Toyota's and though it would have no effect on driving, I'd fail emissions. I thought it over for about a week, then suddenly the light turns off. Thought I'd dodged the problem, but then a week later, the light turns on again. This time, I was worried because I realized that my inspection sticker had a big 7 on it, meaning it was due at the end of July. I waited another week. The light turned off! I immediately took it to an inspection station and passed. A few more days pass and light turns back on. I honestly don't remember whether the light is on or off right now, but I'd like to know how serious a problem this is or if I should just wait it out. I read somewhere that it might have to do with a loose gas cap, so that's the first thing I tried: I bought a new gas cap. Any other ideas? Thanks!