Your Hummer oxygen sensor is a delicate instrument that endures extreme conditions. The sensor's job is to tell the Engine Control Unit how much oxygen is in the fuel. The sensor and the Hummer ECU are in an electrical circuit that remains open until the oxygen sensor is activated by heat. Once a "closed loop" status is activated, the Hummer ECU receives data from the sensor, which comes in the form of an electrical current. Voltage in an oxygen sensor will vary between 0.1V and 1.0V, depending on how rich or lean the fuel is. If the loop is open (or in other words, the electrical circuit is not complete) the ECU reads 0.45V. At that level, the ECU relies on the other sensors to provide data on how much fuel to inject. At one end of the spectrum, the voltage levels will warn the ECU that the fuel levels are too 'rich,' and the ECU will accordingly cut back on the amount that is released. If the voltage levels read 0.2V or less, the fuel mixture is 'lean,' and the ECU will boost the fuel released by the injectors. The Hummer oxygen sensor measures the air levels by comparing the oxygen levels in the exhaust and the environment's air. So when it comes time to replace a Hummer oxygen sensor, take care to select one of the sets made by the brand-name, recognized manufacturers in our site.