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Justin, the code is just telling you that there is what it considers a large leak when the ECM commands the leak detection pump (LDP) to pressurize the fuel evap system.

The ecm decides whether to throw a code for a small leak or large leak dependent on how fast/long the pump has to operate.

Pretty simple system, ECM first checks the LDP for electrical/mechanical fault, if okay it then shuts the vent valve and turns on the pump to pressurize the evap system to check for leaks. No leaks no codes, leaks dependent on size determines the code.

Gas cap is often the problem, followed by cracked/ broken hoses to the various parts of the system, Evap canister leaking, vent valve stuck, and even a leak at the fuel tank.

A smoke test is used to check for the problem. Just a machine that pumps smoke under pressure through the system. Makes it real easy to find the problem, just look for the smoke leaking out.

If you don't want to spend the bucks on a smoke test, time to get dirty.
Flashlight, mirror, mark 8 eyeball. Go looking at every rubber hose in the evap and fuel system. Most of the time you'll find it right in the engine compartment, one or more of the hoses has dry rotted and split.

Should have an emissions label under the hood, shows all the vacuum lines and what they go to. Use it as a guide.

Funny I just had this code in my Colorado, turned out to be the vent valve.
Something like $129.00 for it from GM. Did a little checking and found the same valve is used on the Hummer 3, and it's 1/3 the cost. Wanna guess what I used?

One other thing, A/M gas caps are shall we say sometimes worse at leaking, especially the locking type then what you just took off.
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