A/C lowside won't go above 40psi

I’ve got a 02 Ford Taurus Sel and when I purchase the car recently AC would blow hot air. The compressor would click on and off constantly. So I recharge the system with one small can of R134a and the PSI gauge was reading around 35 or so. Cool air started to come out of the vents and the compressor would stay running. So I added another small can following the directions and it got up to around 40 PSI and was blowing colder air. Not quite cold enough though. So according to the can and the gauge all about 90 degrees it should be between 45 and 55 I believe PSI. So I added almost another whole can and the PSI hasn’t moved one bit. It’s still right on 40 PSI exactly where it was before adding the third can. Again the compressor runs without clicking on and off which is a good thing but why can’t I get the PSI to go up at all on the low side to get the air a little colder that comes out of the vents

Forty PSI is fine. The unknown and likely the critical factor is what is the high side number. What you need is a manifold gauge set so the high side can be monitored. I assume you’re using a gauge on one of those DIY charging cans.

To me anyway, 55 is not a good number because the evaporator temp is roughly equivalent to the outlet air and 55 will not do a good job of cooling on a very hot day.

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With the engine idling @2000 RPM’s/with large fan blowing at condenser.


I’ll run to Harbor Freight tomorrow get a set of gauges. So the extra can that I put in that didn’t move the low side presumably went into the high side and now the high side is probably Way High correct?

Autozone has a gauge rental deal. Why buy them for one time use?

The OP’s vehicle has a refrigerant leak.

They’ll be using the gauges more than one time.



Increasing the low side pressure above x40 degreesx will result in warmer air from the system and possible damage to the high side (condenser, etc) from over pressure.

That should read “40 psi.”


Since you are obiously inexperienced with A/C I might add a few warnings.

Wear safety goggles. Blindness can occur in a second if refrigerant hits your eyes.

Wear gloves to prevent frostbite.

With a full set of gauges and the can of refrigerant being tapped you NEVER open the high side of the gauge set while charging the system. This can cause a can explosion. That can lead to frostbite, blindness, and a ton of excitement when you disappear in a cloud of refrigerant. Keep that high side closed.
I’ve seen this happen a few times and in both cases it was very experienced A/C guys who let their mind wander a few seconds while they made this mistake. Luckily, no permanent damage.


All this tells me you don’t Know how to diagnose A/C systems. Go see an A/C tech before you either damage the system more or worst case hurt yourself and/or others who may be near you.


Hey everybody thanks for your replies.

Hey just so we’re clear I’m an electrician for 20 plus years and my family owns a HVAC business which I grew up around so I have understanding of how an air conditioning system works.

so I got myself a set of gauges and when I initially hooked them up, my low side was reading 38 PSI and the high side was around 175 or so. This is what the car idling and AC on full-blast. At this point the AC was blowing cool air. The low side AC line was forming lots of condensation and was quite cold, and the high side was warm but not warm enough feeling.

Contacting my father and he said the high side should be a little bit higher, to just add some more refrigerant and the high side should rise. But as before when I add more refrigerant neither the low or the high side change but I did notice that the air was just starting to get warmer and the high side line was getting cooler. So at this point I stopped adding refrigerant because obviously there is an issue if the pressure is not changing but the air is getting warmer and the high side line is getting cooler…

Also looking at R134a Automotive charts online show a low side pressure between 40 and 45 and a high side between 215 and 250

so my question is still the same as my original post especially now since I put gauges on to monitor both high and low sides. What would cause the pressure is not to change when adding more refrigerant? Both the high and low side did not move at all. Is it a weak compressor, something with the filter drier, expansion valve?

Your cars AC system doesn’t use an expansion valve. It uses an orifice tube.


The orifice tube screen may be restricted.


Check the pressure with the engine running at about 1800 RPMs.

It’s possible there could be an issue with the one way vacuum valve which can cause issues with the dash controls. They are generally located on the firewall with several vacuum lines attached and one line going into the dashboard A/C heater control. They are about 5 to 10 bucks all depending and considering the age of the car it would not surprise me that one could have failed.

Looks like this…

If the low side pressure is over 50 psi the air coming out the vents will be 50+ degrees.