i just attempted to test my o2 sensor by following the instructions in my bentley book and got stuck. the book describes a sensor signal connector (1 wire) and a sensor heater connector (2 wires). i have 3 wires, 2 white and 1 black, but no 2 wire connector. the black wire had a red plastic thing, and when i stuck the lead from the voltmeter in it, i got readings ranging from 0 to 4 mV (i think thats 0 to .4 volts, i'm not too adept with the voltmeter). so my question is, does that tell me anything about the state of my o2 sensor, and is there anything else i can do to test it? thanks!

The 2 white wires are heater wires. The one black is a signal
wire. 4mV = 0.004V, which means that either your car is running
very lean, or the O2 sensor has no output. Try the test again
between ground and the signal wire with the O2 sensor connected
to the computer. The output should fluctuate below and above
0.6V (600mV) or so.
.
-b.