Air Conditioning Issue - 2016 Traverse

[Ol'Jim]

My girlfriend has a 2016 Traverse that she bought in 2017, with low miles on it.
Has been a very good vehicle, no issues at all, with 97K miles on it now.

Recently, (over the past month or so), the A/C would still cool, but the air was noticeably not as cold as usual on hotter days. Because it is a 3 row SUV, with the ductwork in the roof, it would always take a minute or 2 to start cooling really well, but it would always get very cold fairly quickly. Instead, it just seems to get moderately cool.

I figured it may be low on refrigerant, so I got a recharge can of R-134a with a gauge on it, and the low pressure port reading (while running) was around 24 PSI, maybe a pound or 2 below the minimum / normal range of 25-50 PSI.
I could only get about 2 pounds into it, even with several tries, no more than 26 PSI. That was within acceptable range, and I figured maybe the can was not pressurized any more than that.

Ok, so it seemed to cool a bit better, but still not like it used to. About a week later, she said the A/C would not come on at all, when she pushed the A/C button, instead of it lighting up, the button would flash 3 times then go off. She took it to a shop, and they checked it for leaks, and determined the Schrader valve was leaking, so they replaced it, evacuated the system and recharged it. She said the A/C worked good for 2 days, and now it is doing the same thing, A/C button light flashes, then goes out, no cooling. I have not had a chance to take a look at it, but she brought it back to the same shop, now they are telling her it may be the compressor. I do know the compressor was engaging and running when it was only cooling moderately, and also verified that when I recharged it.

Any ideas? Doesn't seem like the compressor would be the issue, they usually work or they don't, from my experience.

Thanks in advance for any advice on what to look at, or possible causes.

[maxwedge]

The ac system has to be evacuated, properly charged, and checked for leaks using the dye method or leak detection sensor device, then go from there.

[Ol'Jim]

UPDATE: Problem solved.
She had me test drive the car, and the A/C seemed to be working properly.
Made a fuel stop, and upon restarting and turning on the A/C, the illuminated AC button was flashing, and the compressor did not engage.
A glance at the dash, I saw the coolant temp gauge was all the way down at the Cold mark. I knew that could not be correct, as it was over 90* out and it had only been shut off for a few minutes.
Connected my OBD II scanner, and it showed the coolant temp was normal, 190*. Checked for stored trouble codes and found none. While driving it back home, at some point the temp gauge had returned to normal range. Tried the A/C and it worked. A mile or 2 later, A/C began blowing warm air, and sure enough, the coolant temp gauge was back down to the Cold mark .
Did some research, and found that an intermittent coolant temp sensor reading will lock out the A/C to prevent possible overheating, since the ECM is not able to determine the actual engine coolant temperature.
Replaced her coolant temp sensor, and the A/C has been working fine since then, (2 weeks ago).
Interesting that there were no MIL indicators, or DTCs stored. The money she spent on the refrigerant being evacuated and replaced was wasted.