An oxygen sensor measures the difference in concentration of oxygen in the exhaust stream and the surrounding air. It works with a minute quantity of air. Access to the exhaust stream is easy - stick a probe in it. Access to the surrounding air isn't so easy. When the sensor is new, the surface is clean. Drive it a few miles and a film starts to build up. after 10 or 20,000 miles, the outside has enough of a film that oxygen doesn't get to the sensor anymore. So the outside air is sensed by drawing air through the wires from the pcm. Remember all the connectors in the engine compartment have seals on them. And all the wiring is crimped. Air can pass through the crimps and in the small spaces around the wire strands. So the outside air is measured with air that comes from inside the pcm, drawn through the wires and pumped out by the oxygen sensor. The pumping action is accomplished by the heat of the sensor - air around it expands and draws air from the wiring... interesting, huh? If you solder the wires, the solder will fill the spaces that the air moves through and it will set a code for a lean condition...
