How do I prevent the "Loose Gas Cap" error on a 2004 Buick LaSabre from setting the Check Engine light, and causing my car to fail emissions testing?

Only if the gas tank has a leak in it. Try a new gas cap but anything in the EVAP system could be a cause of your problem.

In regard to the answer give by Full_of_Regrets above, if the gas tank had a leak, wouldn't it lose fuel? I have never noticed any leakage of gasoline around my tank... Or do you mean a leak above the highest level that the gas reaches when the tank is full? I guess excess fumes could go out of a leak that above the level of the gasoline? If such a leak was causing a problem, is it possible to detect the leak and seal it, or something like that?

Replace the gas cap. Not likely you have a "leak" in the gas tank, but have the mechanic you used tell you why he is recommending the tank be replaced. Sounds like he is taking advantage of you but ask him to prove his point. There are several other things to check before suspecting the tank. It ma simply be a disconnected vapor return line or plugged evap collector line. Good luck and think about getting a second opinion.

If there were a pin hole in the top of the tank it could cause a problem with the EVAP system but this is highly unlikely.

Try gas cap, followed by having the seal replaced for fuel pump. Both of these can cause a "vapor" leak and will give you that light. I had to replace both of mine before the light would stay off.

1. gas cap 2. leaking EVAP vacuum (harness connects from throttle body to EVAP purge solenoid and fuel pressure valve) 3. bad or sticking EVAP purge valve 4. leak between EVAP purge valve and canister 5. bad EVAP canister 6. leak in fuel filler tube between gas cap and tank. 7. (and least likely) leak in gas tank.

I have a 2005 buick lesabre light on dash gas cap loose its new and tight this should be a reacall

I replaced gas cap on mine, and sanded the roughness off the top of the filler pipe to create a better seal between gas cap and filler pipe, light went out..