Where is the AC Low pressure valve on a 1999 Pontiac Grand Prix?

Is the compressor turning on an off quickly ?
Could be a low pressure switch or problem with the ac clutch.

A/C pressures when running? Low side 35-45 High side 225-275

Follow the lines from the A/C compressor across the engine compartment. The low-pressure port is the smaller one. They're near each other.

Look for a low pressure switch near,or on the dryer,it may have went bad,also have the freon checked ,if it is to low,or out,it will not come on.

If the temperature gauge looks good (under 3/4) its all good. If not, then there is a problem. If it smells hot inside, like moist and humid you will need to replace the ac filter, that can be located under the hood, close to the water resevoir, you will have to pull the ruber type gasket that is on the edge of the hood/wiper cover.

The sensor is located near the hood latch area in front of the radiator. If the temp is reading incorrectly, chances are it is defective. You can purchase a new one, they relativly inexpensive.
Disconnect the electrical connector then remove the sensor.
Good luck, let me know.
Regards,

Under the hood towards the front of the engine compartment is the condenser. This has two lines on it. One is larger than the other. The larger line is the low pressure line with a blue cap on the service port. To fill the freon you must get a can with a hose that hooks up to this fitting. While the a/c is on max, open the valve on the freon(134a) can and the compressor will start to run. When the line starts to sweat check the temp coming from the vents, the white sticker under the hood will tell you the a/c system capacity. Don't overfill it. Good luck

that light goes on mostly likely cause you need freon. the compressor will not go on if there is no freon. buy a userfriendly can at autozone. put it in low pressure valve(blue)  if it works but no cold enough air check under your hood for the proper pounds or ounces needed of freon.  if it doesnt work its your compressor.  GOOD LUCK! :)

Not sure what year your car is but on 2004's the low side is low behind the engine on the pax side. Near the alternator.

I think that there is a button located under the dash at the right side of the steering column. To reset the low tyre pressure warning you need to press the button immediately after inflating all tyres to right pressure.