1997 Dodge Dakota PO132 code

The O2 sensors can't generate a voltage much higher than 0.8 volts. To get that code is almost always due to a wiring problem as in the signal wire and the 12 volt heater wire are shorted together. The easiest way to figure this out is to connect a scanner that can read live sensor data. View the signal voltages for all of the sensors to be sure you're working on the right one.

The heaters are already grounded to the body so that wouldn't be an issue, but the sensor circuit can not be grounded. They go to ground through a circuit in the Engine Computer. That's how it can measure the integrity of that circuit. All four O2 sensors are grounded on the same pin so I don't know what would happen if only three made it back there and you had one sensor grounded on the body.

As for the 4 volts, there are two possibilities. With other sensors the computer uses "pull-up" resistors on the various sensor circuits. They are so high in value that for all practical purposes they aren't even there, but when there is a break in the circuit, the computer will see the voltage coming from that pull-up resistor. That forces the computer to recognize a defective value and set a fault code rather than allowing the voltage to "float" around at some random value. Normally the computer will see full battery voltage or 5.0 volts from the pull-up resistor. The point is it won't have to guess or be confused.

The second possibility is that 4 volts was indeed some random value coming from other circuitry in the computer in the absence of a strong proper signal from the O2 sensor.

My guess is it set the "voltage high" code, which there was no arguing with, instead of an "open O2 ground" code because it was still seeing the ground current flow from the other O2 sensors. It would likely have set an open ground code if the ground wire was broken before the splice where it branched off to all of the sensors.

Happy to hear you found the problem.