A monitoring system for a heating, ventilation, or air conditioning (HVAC) system of a residential or commercial building includes an evaporator unit device and four temperature sensors. The evaporator unit device includes an electrical sensor that measures current supplied to a circulator blower of the HVAC system. The measured current from the first electrical sensor is used to diagnose a problem with the circulator blower. The first temperature sensor that measures a temperature of refrigerant flowing between a condenser of the HVAC system and an expansion valve of the HVAC system. The second temperature sensor measures a temperature of refrigerant flowing between an evaporator and a compressor. The third temperature sensor measures a temperature of air flowing away from the evaporator. The fourth temperature sensor measures a temperature of air flowing toward the evaporator. The evaporator unit device transmits sensor data to a remote monitoring service over a data network.