Auto Range switches through ranges and finds the best one for the current measurement it is checking. NaN indicates over range. If the device Auto Ranges to a single range and is used for multiple measurements, but the signal rises outside of that range, the DMM will return NaN.
- Evaluate the values to be expected (if reading multiple voltages, know the values of those voltages)
- Set range to the largest voltage expected
The caveat to this is less accuracy. The larger the voltage range, the lower the resolution of the measured signal.