Often times people think that refrigeration is more complex than it really is. Yes, there are a lot of variables at play; but if you know what to look for, often times the problem will reveal itself. Key elements of a system include pressure and temperature, as both of these are directly related. External factors, such as poor ambient conditions or a dirty condenser coil will affect the efficiency and possibly the operation of a unit. So initially when a unit goes down, check for mechanical operation. By checking this it will determine what direction in which the service call will go. If it is not running then you can start at the electrical components and work from there. Diagnosing mechanical problems will often be a bit trickier. As a technician, you need to think in terms of temperature, rather than pressure. Ultimately, pressures vary from a range of different systems relative to the application (ultra-low cascade units run in a vacuum). The laws of physics do not change. Superheat and Subcooling are what determine the efficiency of a unit and will generally give a clear cut explanation as to where the fault is.