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It could be the vent solenoid (part of the evap system that seals the system to check for leaks) doesn't seal properly all of the time. That could give you an intermittent leak.

It could be the purge solenoid (part that creates a vacuum during testing) doesn't seat properly and is causing an intermittent leak.

Also the gas cap can be the cause because even though it passes the leak check, it may not seal properly every time. Does the problem happen after fuel ups? If you decide to change it, I recommend buying one from the dealer as aftermarket caps sometimes do not have the best quality control and can leak right out of the box.

And it could be caused by the fuel tank pressure sensor. It is the sensor that is setting the code.

Basically on most GMs the car is started dead cold, the vent solenoid seals the system and the purge solenoid create a slight vacuum in the tank then closes. The FTP sensor measure how long the vacuum stays. If it goes away immediately, it see that as a leak and sets the code. If it holds if for a specified time, it see that the system is sealed and passes the system.

If it was in my shop after it passed the smoke test, I would get a new cap and see what happens, if it still set, I'd change both solenoids and recheck. Evap leaks can be VERY difficult to find and if those items didn't fix it, the next step is to drop the fuel tanks and visually inspect the line to and from it. When an easy diagnosis isn't possible, I always start with what is least expensive to the customer and what are the most common failures.

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Update: I took it to another garage and they replaced the vent solenoid,but the engine light came back on. I'm taking it back for a follow-up.

I'm not sure if it is related,but when I park the car, sometimes I hear swishing in the gas tank. It happened today when the check engine light came on,just after I stopped the car and shut off the engine.

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