OK that year should use the Natural Vacum Leak Detedtion switch system. I dont have the diagnostic book for it as they are nice and help alot to a specific code. From what I remember what happens is 2 things. When you shut the car down after driving the fuel will cool down some and pull a small vacum in the tank and then the NVLD switch will close and that tells the PCM its working ok. Then when the car is running in the right parameters the purge solenoid opens and purges the gas tank for a few seconds and pulls a larger vacum on the gas tank to see if it will pull the NVLD switch closed and if it does it will turn off the purge solenoid and see how long the tank will hold the vacum before the switch opens. Thats how it detects how large of a vacum leak it could have by how long it holds the vacum.
Both of the test like all OBDII test have to meet certain parameters before they run. Since the PCM moniters the switch wire it will know if its state changes from high volts (open) to low volts (grounded) and from what I remember if the switch volts does not change on either it sets the switch code thinking something is wrong with the switch or electrical circuit. It can also be a very large vacum leak in the tank and hoses but it only takes a very small amount of vacum to close the switch so it would have to be a good size leak to fail the second test. With a scanner you can moniter the switch and pull the connector and ground it to test the wires and then comand the eng to purge and pull a vacum to test it. Of course look for any cracked hoses or vacum leaks as they might cause it but most of the time that code can be a wire or NVLD switch problem. It would be nice if you know someone who will scann it for you if you cant find any visable leaks. Ron