Context:
AC has been recharged twice in the last ~5 years (never investigated a leak - long long story, don't ask).
The other day it was blowing pure hot air, so I thought it was low again. I bought this r134 UV kit with a gauge and a light pen to see if I could find the leak before taking it in -- just so I could not get rolled over on.

So, I read the instructions, windows/doors open, AC on full blast, hooked the gauge up to the low pressure and noticed the gauge was fluctuating from way...

a/c compressor going on and off (2006, 4.6L)