The internal circuitry is self-powered by the induced current from the monitored line

The 391937 and 391937/U are the same Honeywell/Resideo conversion kit

independently-adjustable SPDT snap-acting switches mounted on a single foot bracket

By utilizing a direct-coupled mounting configuration

Built to exact OEM specifications

Carrier HH79NZ039 Sensor 322027-701 The internal circuitry is self-poweredThe Carrier HH79NZ039 is a Factory Authorized Part designed for high accuracy thermal monitoring in commercial and residential HVAC systems. This versatile thermistor is primarily utilized as an Outdoor Air Temperature (OAT) sensor, providing critical data to the economizer logic to determine free cooling eligibility. Beyond ambient air sensing, the HH79NZ039 is engineered for multi mode service, functioning effectively as a Supply Air Temperature