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