outputValue = ((long)sensorValue - 512 ) * 73.982L;
Well this line makes no sense at all.
I concur that 0.4 A seems like a large current to be reading when there is no current there.
But this may be just an offset error in the device, which you can account for in your code,
if it is linear.
To check that your device is actually working, I suggest you figure out some way to put
a known small current through it, and see if the device detects its presence.