you use a scanner, usually the tool you got the code from, so see what the FTP sensor is doing. you have a 5V reference on one wire and ground on the other side wire. the middle wire is the signal back to the computer. At atmospheric pressure the FTP should be very close to 0, zero. the give you an offset, (I don't know why they went to use this term, it's just another way to say: allowable variance) of +/- 1.7 inches water. So if your gas cap is off, the FTP should be 0. The code you have says that the sensor signal is erratic. Moving to much in a short time. It could be a loose connection or an intermittent short to ground. It could be a bad sensor. Without a scanner or a volt meter, preferably a graphing one so you can see what is happening, hooked up to your car and see what the signals are doing when you are driving, to know what the problem is coming from you are reduced to replacing parts and see if that takes care of your problem.
the system is sometimes under vacuum and sometimes not. There is a vacuum switching valve that does allow vacuum into the system. However when that valve is closed and the vent valve is closed there may be a positive pressure condition. With the car stopped and sitting for a time the vent valve should be open and the system should be at atmospheric pressure or neutral.http://www.youtube.com/watch?v=i0e-Uu4fXTs