Methods of monitoring operation of a refrigerant-cycle system are disclosed. The methods include: measuring a first pressure of refrigerant input to a compressor of the refrigerant-cycle system using a first pressure sensor; measuring a first temperature of the refrigerant input to the compressor using a first temperature sensor; measuring a second pressure of refrigerant output by the compressor and input to a condenser of the refrigerant-cycle system using a second pressure sensor; measuring a second temperature of refrigerant output by the condenser using a second temperature sensor; measuring a third pressure of refrigerant output by the condenser using a third pressure sensor; and measuring current to the compressor using an electrical sensor. The methods further include identifying conditions of the refrigerant-cycle system based on the first temperature, the first pressure, the second temperature, the second pressure, the third pressure, and the current.