1996 Ford Mustang Engine Light comes on - Engine Not the pr

Hello, My son has a 1996 Ford Mustang. The engine light comes on periodically. Someone tested it with some sort of tester, and said that the problem is the Evaporation Canister perge valve, something to do with the exhaust. The guy with the tester, reset it and the light went out yesterday. Now today the light came back on. We are needing a picture of this part, so we can find it, and look at it to see what the problem is.
Thank you.