I had my "check gas cap" notice show up on my 2008 lacrosse dash. I checked the cap and cleaned it off, The notice appeared the next day so I removed the cap and covered the opening with a plastic bag securing it tightly around the opening with rubber bands. The next day the notice appeared along with the check engine symbol. I checked the plastic covering on the fill nozzle expecting to see the plastic being sucked in a bit due to the vacuum in the tank but the plastic wasn't stretched. I checked the other connections and they appeared to be OK. I then removed the purge valve and was able to blow air through it from both sides. Would this be an indication that the purge valve is malfunctioning? I hate to just start replacing parts at random.
I have no specific knowledge on the 2008 LaCrosse, however --
Be careful with your assumptions. The purge valve is usually under the hood and is normally closed when not energized. The vent valve is usually in the immediate vicinity of the charcoal canister and is normally open when not energized.
When the ECM decides that it's time to purge the charcoal canister, it closes the vent valve at the canister, then opens the purge valve under the hood briefly and intermittently, applying manifold vacuum through the purge valve, through a tube to the canister. The reduced pressure in the canister allows it to give up fuel vapors to be burned in the engine.
When the ECM ends the purge, or when the engine is turned off, the vent valve near the canister opens to the atmosphere and relieves the partial vacuum in the canister.
NOTE: A plastic bag and rubber band on the fuel filler, no matter how tight, will not hold pressure or vacuum sufficiently to pass the evap self-test.
OK, so utilizing information from @MTA about the actual operation of the purge valve, I applied 12 volt DC to it and it operated but never sealed when not energized. This confirmed a bad valve. I took the valve apart, destroying it. In the picture below, you can see the valve base on the left where the valve would seal and on the right, the plunger. You'll note the smooth surface on the right and the darker, rough surface on the left causing it to leak all the time.
I replaced the valve and cleared the codes and the car is back to normal.