I have a 1999 isuzu rodeo lse with the code po452 evap emission system press.sensor low. What does that mean and where is it located on the car.

I was told to first buy a new gas cap,and run it a few days,to reset codes and if it still showed up the señor is located on the top of fuel pump,will have to lower gas tank to replace.good luck... I just had a lot of work done to my 99 rodeo and now I am getting code p1640...