Oxygen Sensor

The oxygen sensor is mounted in the exhaust manifold. It measures the amount of unburned oxygen remaining in the exhaust gas and signals the ECU to change the amount of fuel being injected as necessary to maintain the proper fuel/air/mix.

There are two times when the oxygen sensor is cut out of the circuit. When the throttle is closed (idle), since the best mix, from both an emissions and smoothness standpoint, is leaner than a running mixture, the throttle position switch cancels the signal being received from the oxygen sensor at the ECU. A second time when the oxygen sensor signal to the ECU is diverted (also by the TPS) is under full throttle conditions, when a slightly over-rich mixture is required to keep exhaust gas temps down, protecting the catalytic converter and the oxygen sensor.
The oxygen sensor operates under a very low voltage, and therefore, current, either from a test instrument, should never be applied to the wire. A voltage can be measured (less than 1 volt) from the sensor but it must be read with a high impedance digital meter. The oxygen sensor does not function until it reaches a rather high operating temp.