'91 2wd 3.0
I'm trying to chase down a rich running condition. Have done pin tests per the FSM on damn near every sensor. All check out OK. Next step is 02 sensor. I went ahead and ordered a new one and was going to test that.

First weirdness -- the FSM shows the O2 sensor having a female connector. The Denso 234-4055 I got has a female connector. But the NTK that came off the truck has a male sensor. The truck-side is the female connector.

Is that normal? Or is it possible the prev owner swapped the harness? Should I convert the harness over and use the new Denso?

Next, I went through the FSM procedure on the existing NTK sensor. You jumper the diagnostic port same as getting codes; then put voltmeter on two terminals and run at 2500 rpm. The voltmeter should "fluctuate" 8 times or more in 10 seconds. I get 2.33 volts, and it fluctuates by 0.01 volt.

Is that a "fluctuation?" Seems like an awfully tiny movement.

If we assume, No that is not a fluctuation, the next step is to test voltage between the two check terminals without the code jumper. The choices in the FSM are "zero" or "five" volts. I get 2.33. So...WTF do I do now? It's not zero or five.

This is all proving maddening.

 

When I was trying to figure out my oil gauge issue the FSM had me do a similar fluctuation test at the sender. The FSM, being written back when quill pens were still used, showed an analog multi-meter. I was using a digital meter. I could never get a good fluctuation reading. I always wondered if the different styled meters was the cause. It turned out I had to repair the oil gauge itself, and now works great. So when i was testing the sender It should have fluctuated like the FSM displayed.