Why do I have low air presure - Cars & Trucks

There are 2 lines coming from the firewall on any vehicle that has ac. One is the high presure the other is the low presure. Follow each line until you find fill valve.

This valve is used to fill the system. It will be covered by a plastic cap that screws on. Each side, the high and low has one of these valves. Often the cap is labeled with an "L" for low or "H" for high. Another way to tell is to flip the caps upside down. The low presure side will always be the smaller of the 2.

could be low fuel presure /mass air flow sensor /or restricted catalist system check exaust back presure

Sounds like you might have a leak. If you charge the system and works for a time ,(hour, week, year) and then quits it sounds like perhaps the freon is leaking out somewhere. Freon does not ever get converted to air. Freon is in several states of existance while it is in your system. It is a high presure liquid, high presure gas, low presure liquid, and a low presure gas, but it is still always freon.
The valve your friend is talking about is called an expansion valve. What it does is to take the high presure "liquid" freon and allows it to pass into a low presure zone called an evaporator at a regulated rate. When you take freon at a liquid state high presure condition and expose it to a low presure environment it "evaporates" ( thus the name evaporator) and it gets very cold when it does this. This is where the cold air you feel coming out of the vents is made.
But the freon is in an enclosed system. By this, I mean it is never exposed to air, moisure (water), or anything else. I goes through all of these changes while inside this enclosed system.
Therefore, if the enclosed system is good then the amount of freon inside it will not change. If the amount of freon doesn't change then adding more will only make things worse.
I would suggest having the system checked with a hologen leak detector and/or system dye to find the leaks. Remember, if you have to add freon, then the freon that was there before must have escaped from somewhere.
Find a reputable shop to check the system for leaks. If it is leaking bad enough to run low in a hour it should be an easy to find leak.

Did you check spare tire in trunk?

i think i would check the fuel presure regulator and the fuel presure if fuel presure low look at fuel filter if rail not holding presure then look at fuel presure reg and vac conection on it

Yeah Its a tire pressure monitor. When the temperature decreases, the tire pressure light comes on if they are just a little bit low. I filled mine up to the proper PSI and it stopped the issue.

has the charge been tested< these suffer from leaks, have the system leak tested and then repair and recharge.

check your orifice tube if its plugged up it will give you a high reading.

if refrigerating system low in presure(low refrigerator level)presure sensor wil not send signal to engage clutch...the second think if system under normal pressure check electrical connectors if they ok,a/c compressor is faulty.

problems yes low high and high low are usually caused by faulty defective clutch coil or defective thermostat...electrical issues can be cycling switch pressure switches ambient air sensor or fin switch ( evap temp sensor)
Mechanically it could be clutch,or compressor the symptoms are not good... srry hit me back if you got some manual gauges and wanna run pressure test diagnostics..