engine light on and keeps telling me to tighten gas cap

I have had this problem for a year now. I have changed the cap 3 times. 2 from auto stores and 1 from Chevy dealer. I just had a switch sensor go out and the mechanic ran all the sensors and nothing on the engine light showed up. A day after having the truck repaired the "gas cap is loose again came on and the engine light came back on. I am not able to replace the solenoid and why doesn't it show up on the sensor at the repair shop.

Hooked scanner up to truck 2006 Chevy Silverado and the error code said loose gas cap. Which I found always mean a bad Vent Solenoid. It was a easy fix! Order a new vent solenoid from Amazon due to price, put it on " Check Engine" light went off no more problems. Just want to add that some vent solenoid are located in a hard place between cab and bed. In that case I had to loosen the bed and jack it up so I could get to it. Hoses: You need to make sure they are not cracked and are tight and the system is closed so no air can get in except for what the Solenoid Vent lets in. Hope this helps, good luck