Where's the AC low pressure valve
on passenger side under hood,look for a blue cap starting at firewall to front .
Its normally on the back of the motor on the passenger side. My a/c guy has to reach his hand a good ways down into the motor to get to it so yours might be the same...
well my sister has a 98 with the 3.1, its on the front right hand side, on the bottom of the motor, it should be right on the line coming out of the a/c compressor, it was a little hard to find till i googled it, the 4 cylinders should be on top of the motor right hand side, my 95's is, hope this is helpful to you, good luck.
OK After reading and looking some said it was a green cap some said a blue cap some said it was up front some said it was on the fire wall.. so the answer is , Its a blue cap and you cannot see it it !! look directly in the BACK of engine in the center , look all the way down you will see 2 aluminium large line , reach your hand down and follow the line it will curve around a bit and you feel it!! but wont see it it .. just unscrew and plug the refill line .. Have an Awesome day ! 2003 Grand Am 6 cylinder!