I found the following quote in an old thread by Barrie. What does he mean by "shorting the input"?
Testing Sensor Element: measure the output voltage of the sensor ceramic (black wire) under full-rich conditions; create this condition by shorting the input to the ECU (green wire). Specified tolerance is 0.45 - 1.0 VDC. Mine measured 0.48, suggesting that the ceramic element is getting weak.