2003 Tahoe emission code trouble

Here is the situation: My wife has an 03 Tahoe that has just shy of 50K miles on the clock. It has been a great vehicle until recently. The engine light just came on one month out of GM's 3 year warranty. I scanned the computer which gave me two trouble codes. They are both P0440 evaporative emission control failure. One of the codes was a pending code meaning that the problem has been there for awhile. So she took it to the dealership where she learned that the GM warranty just expired, no problem because she bought the extended warranty, right? Wrong. The extended warranty did not cover the sensor which they determined was the problem. It is the fuel tank pressure sensor which is located on top of the fuel tank at the sending unit. They wanted $500 to change it. Yeah, right. It took me about two hours working on my back to replace it. Well the problem apparenty still persits because the idle goes really low at times and it seems like it wants to stall. The check engine light came back on now as well. I am going to scan it again tonight. It looks to me that there are 3 sensors responsible for the evaporative emission controls; the one I mentioned, another one at the rear of the tank, and one on the intake toward the front which is connected to the tank vent line. It seems funny they found the one which required the most amount of labor that needed replacement. Anyway I hope someone can shed some light on my situation. Thanks in advance, Jeff

Emission trouble code

I was incorrect about the code number. It is P0446, emission vent failure. I am hoping it is the gas cap, however, the car runs like it has a vaccum leak. It will idle okay and then it will drop down to really low rpm and almost stall. Anyone else have this trouble code? Jeff