When measuring between the signal out and common leads, the ohmmeter should measure infinite or at least above 50 megohm. Sensors with reverse power protection use zener diodes to shunt the reversed power. Thus when the power is reversed the zener voltage is measured. Sensors without reverse power protection do not usually survive mis-powering and are permanently damaged. Common sources of sensor damage are exposure to excessive temperature, excessive shock, mis-powering, and electrostatic discharge.
Excessive temperature is the most common cause of sensor failure.
Sensors caught in a fire are usually destroyed and can show various bias readings depending on the failure mode within the sensor. Long term temperature failures are marked by a slowly declining bias voltage. In many cases bias returns to normal as the temperature decreases and the sensor can still be used in lower temperature applications. Figure 5 shows the bias trend of a sensor failing from long term temperature degradation in a paper machine dryer section. Note how the bias voltage increases when the machine cools during machine shut down.