Bought one of those refrigerant cans that come with the gauge. A month ago I filled it to the normal pressure and it blowed cold air for once. The next day it was blowing hot air as always. Today I went with the remainder of the refrigerant and filled it back up. It read about 40 PSI and still blew hot air. I turned the car back off and on and the pressure read close to 0 PSI. I filled it to about 20-30 PSI with what was left in the can and there was no change. I turned the car off and on again, thinking it might reset to 0 PSI like it just had, but still read about 20-30. Still blows hot air. What could cause this?