Variation in the coolness of the air. Some days it works fine, then it blows warm and vice versa. Since it blows cool some times I assume the charge is adequate. Any ideas - temperature sensors, etc. - I should look for and where they are located?
Thanks!

 

Measure system pressures anyway (high & low). There's pressure switches that shut off the compressor when (for example) the pressure is too low. If your refrigerant charge is marginal, it's possible that a pressure switch is sometimes allowing it to run, sometimes not.