Nice, Sam! I'm going to run a few tests on mine with the Tech 2; I'm thinking that I somewhat understand the system. I was able to pull vacuum on the whole system a month ago, just messin' around, so I know the purge valve opens and the vent valve closes. Just not sure if one is intermittent, and I know the purge valves can be. Just need to be sure that the tank pressure sensor is working. My purge valve is probably OE. Flex calls for an AC Delco 214646 purge valve -- if you remember, is that what you used for yours (since I know your Yukon is Flex)?