The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100mV (high oxygen content - lean mixture) to 900mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give the injectors. For example, if the PCM reads a low HO2S voltage, it senses a lean mixture and commands more fuel. If the PCM reads a high HO2S voltage, it senses a rich mixture and commands less fuel.