I need to add refrigerant to the AC system in my '02 Suburban but I can’t find the LP port. Where the heck is it?
So, how could you possibly know that the system is low on refrigerant? You couldn’t have connected gauges to both the low and the high ports if you can’t find the low port.
I bought a can of refrigerant with a pressure gauge port that needs to be connected to the LP port. Will this not indicate whether I need refrigerant?
It’ll only give you a guess at it. If there is some other problem with the system, then your low pressure only reading can mislead you.
With modern A/Cs we generally recommend not making this a DIY fix. Far too many errors and some possibly dangerous are made by the DIYs While I did do some such work in the old days, I would not do it myself today, nor would I have anyone other than a properly equipped and knowledgeable professional do my work.