A loop powered process instrument comprises a signal processing circuit measuring a process variable and developing a measurement signal representing the process variable. A control system, for connection to a power supply using a two-wire process loop, controls current on the loop in accordance with the measurement signal. The control system implements a diagnostic function comprising selectively controlling loop current at first and second select current levels and measuring terminal voltage at each of the first and second select current levels to determine if power supply voltage is at a select voltage level.