For o2 sensors??
O2 sensors read the amount of oxygen in the exhaust. They do that w/ an chemical reation that produces electricity... that needs heat, o2, and zirconium IIRC.
One wires have a signal wire and are grounded to the exhaust w/ the housing
2 wires have a signal and dedicated sensor ground.
3 wires have a heater +/-, and one signal wire
4 wires have a heater +/-, a signal, and a sensor ground.
The heater is there to bring the sensor to temperature faster so it can accurately analyze the exhaust gasses and tell the PCM how to deliver fuel/ timing quicker.
To test it, backprobe the signal wire, and rev the engine for a while to make it hot. Pull up the pattern on an oscilloscope, it should look like this.
Should have about 10 peaks/valley per second. If it only have like 5 or less then it's getting lazy and doesn't read as well, which happens.
If its mostly high, you have a rich condition where too much o2 is being burned.
If it's mostly low, you have a lean condition, which may be from not enough fuel being delivered, incomplete burn, exhaust leak, vacuum leak, etc.
It may be broken and not able to see lean/rich. In your case, it may be stuck on lean.
Inject some propane into the intake, and look for the graph to spike up to rich. That proves it can see rich.
Pull a vac hose, look for the graph to hit the basement, that proves it can see lean.
If it passes those the O2 sensor is ok.
However, depending on the graph you see, we can help you furthe diagnose what's going on.