Evap code on my 99 GMC Sierra?

[vdd88]

My mechanic told me i should try replacing the gas cap first after he scanned the code
I got a new cap, but a week later the check engine light came back on
What could it be?
He said if it came on again he would have to do a smoke test - how much will it cost me?

 

[alno1995]

You have a leak somewhere in the Evap system. Smoke test is the best thing to do but expensive. What I did was replace a $17 evap vent valve. It lets the engine vacuum pull the excess fumes back into the intake. I got lucky but $17 is not a bad investment, it was dirty and you could possibly clean it if you can get it off the truck. Dirt could stop the mechanical part of the valve from functioning.