AC was working great last year and suddenly this year the cooling is very poor. Noticed the compressor was never turning on. At first I suspected a leak so I used a can of coolant w/UV die and went through the whole system front to back and found no leaks. I then topped off the coolant until the pressure on the low side was within spec. Compressor kicks in now.... good.
But it never shuts off. Additionally, I hear a kissing under the dash that I never noticed before.
I then suspected that perhaps the pressure transducer was bad (never tells the computer the pressure is high enough) so the system over pressures and coolant escaped from the overpressure valve near the evaporator. So I ordered a new transducer and a set of pressure gauges so I could see what the system was doing.

Replaced the transducer, attached the gauges and let it run for 5 mins or so. Temp outside reads 60'F. Compressor still stays on, hissing noise still evident and low side pressure reads about 20psi while the high is 200psi. I should expect the compressor to eventually shut off no?

Is there anything I'm missing?
Thanks!!

 

Sounds like the evap has a leak. That would account for the missing refrigerant. Best is to use a sniffer at the AC vents to check if there is refrigerant leaking.