A diagnostic circuit is configured for connecting to a unit under test that has a load and a sinusoidal source. The diagnostic circuit includes a voltage sensing device that has an input for sensing a signal, a first terminal for connecting to the load, a second terminal for connecting to the sinusoidal source, and a relay connected between the first and second terminals for connecting the sinusoidal source to the load. Clamping diodes are provide for clamping a sinusoidal signal and include a first clamping diode connected between a D/C voltage source and the input and a second clamping diode connected between ground and the input. A resistor is connected between the D/C voltage source and the first terminal. The diagnostic circuit verifies the operational functionality of the load, related wiring and connections.