Re: 98 Ranger XLT 4 cylinder 2.5L o2 Sensor
I am pretty sure the O2 sensor plugged into the exhaust manifold is the one used by the computer to adjust fuel trims. There may be a second sensor after the catalytic converter which is used to tattle on the converter being effective or not. It would report converter efficiency below standards or some such message.
The upstream(in the manifold) O2 sensor should be wired, and as I understand, all OBD-II computers would throw a code about the sensor being missing or inoperative. OTOH, if some 'technician genius' decided to jumper the O2 sensor wires and produce a fixed output to fool the computer, the wires may terminate far from where they should.
There should be a ground, heater, sense and 'base 5V reference' conductor to each O2 sensor. I think.
If you have a replacement sensor, look up higher in the engine compartment for a set of melted wires or a melted connector, or wires tied out of the way by zip ties or wire.
I suspect it could be atop the bell housing, or up on the firewall. If not, go to a dealer and find a similar model which you can inspect to see where the wires route, and what they look like.
tom
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