yekimmrt you are awesome! Thanks for the great write up!
I got P0446 code on my 2002 GTP 40th anniversary edition. You have the best explanation specific to the grand prix that i could find anywhere. I went ahead and checked the Vent and Purge solenoids with and without 12V applied and they worked as they should. The Vent Solenoid behind the driver rear wheel well cover had a ton of dust come out when I blew in it, but my issue also ended up being the Fuel Tank Pressure Sensor.
I used your idea back probing the middle wire with a safety pin and made sure I had a good ground by running a jumper wire to the battery negative. I was only getting 0.8-0.9 volts on the multimeter. After watching some South Main Auto Videos on Youtube about GM Fuel Tank Pressure Sensors going bad, I knew I should be getting 1.5V. I ordered up a new ACDelco 16238399 GM Original Equipment Fuel Tank Pressure Sensor from amazon for $35 bucks and popped it in. The new sensor showed 1.52V. PERFECT! FIXED! no more annoying light.
