Just tested the O2 sensors in my 94 Z28...i am supposed to (according to my Chiltons book) get a reading of 350-500 millivolts (with key on engine off), but i am getting a reading of 1.5 millivolts while engine is off and key to on position...are the 02's just bad? Or am i testing wrong? I followed the chiltons directions to the T and am guessing the sensors are just bad...i believe they are the originals...with 58,000 miles! BTW both sensors gave the SAME exact reading?!?

 

If you're using a volt meter, I hope its a high impedance (50Kohm min) digital meter. Anything else can damage the sensors. Even with the right meter, you will not get reliable readings, the error can be up to 300mV. You need a scanner to measure them accurately.

Which wires are you probing? Is the sensor hot (engine was running) or cold (engine off for a while)? How long did you allow the heater to operate before you measured the voltage?