Disclaimer: I'm not a car guy. I have a 2004 Grand Prix GTP. I have 105K on the vehicle and I'm original owner. In the past few months, I started getting evap codes. The first time was, I believe the P0440 code and my info center said check gas cap. I changed the sensor/canister (AC Delco) that is located in the LR wheel well and I changed the solenoid sensor (AC Delco) at the engine. This was maybe about a month ago and I did this myself.
CEL just came on again and I now have the P0496 code. According to the Auto Zone scan print out, it appears that the only thing I have not changed is the Evap Pressure Sensor.
So knowledgeable people, where is the pressure sensor located on my GTP?
Should I start with replacing that part first or should I check/do something else first?

Correction previous code was not 0440 it was 0455

Welcome to the forums!
When I looked up PO455 this is what I found:
P0455 Pontiac Evaporative Emission Control System Leak Detected Gross Leak OBDII Engine Light Trouble Code | Engine-Codes.com
As this really is a large leak detected, did you replace the gas cap with a new OEM one?
Find any cracked lines that might cause a leak ?

I am sorry, I didn't make that clear 0455 was the previous code where I replaced the gas cap, solenoid and canister. The current code is 0496

This site also mentions purge solenoid & has good P0496 diagnostic information & tests.
P0496 EVAP Flow During A Non-Purge Condition

Anyone else got some help in this P0496 situation?

Soft Ride, he says he replaced the purge solenoid.......from his description of the rear, it sounds like he also replaced his vent solenoid.....

The code, P0496, usually means a vac is present, when there shouldn't be....so either the purge solenoid is not sealing(but he replaced it), or the vent solenoid is not opening when commanded, or there is a possible blockage......

Basically, he has become a parts changer, and I guess he wants us to recommend another part to change......his pressure sensor is located on the top of the gas tank....I believe on this year car, he has access in the trunk to it(remove carpeting and then the cover plate).......so the tank does not have to be dropped.....

Basically, the car goes through a system check.....it will close the vent solenoid in the rear , open the purge solenoid, and look for a corresponding drop in pressure(increase in vacuum)......then it will close the purge solenoid, and watch to see if the pressure changes in a prescribed amount of time......when it wants to purge the system of vapors, it will open both solenoids(this brings in fresh air o revove vapors from the charcoal cannister)......during all these tests, the pressure sensor data is monitored by the PCM, for the appropriate data with each action.....

Of course the only way to know for sure is with a bidirectional scanner, like a Tech II....you run the car at idle, you close the vent and open the purge and look for a drop in pressure.....you then seal the system by closing the purge solenoid and see if the pressure is maintained......then you open the vent and see if the vac in the system drops.......

Now the normal fix is replace the purge solenoid(leaking vac)....but he has replaced that, and assuming that it is good.....another possibility is the harness to the solenoid.....the ground side is "pulsed" by the PCM....however, if there is a a rub though condition or a wire that is exposed due to a rodent eating the insulation, then that solenoid could be on, without being "commanded", and could set the code....so check the condition of the harness to the purge solenoid......

If you have a vac gage, you can check the purge solenoid(I know it is new)........disconnect the harness from the solenoid......disconnect the lines from the solenoid(2)....attach the vac gage to the line from the intake....should read around 18".....reattach that line .........now attach your vac gage to that open end of the solenoid......should read no vac at all......now rev the engine several times to 2000 rpms and look at the reading.....should still be zero......if you get a reading, the solenoid is not sealing and is no good....

The other possibility is a PCM problem, but that would be very rare....

I will get on checking the things above and see if it helps. I will keep you updated just for fyi. Thanks again for the help, Soft Ride and Tech II, much appreciated.