TECH FEATURE Air/Fuel sensors (also called “wide band” O2 sensors) work differently than traditional zirconia O2 sensors. They include an “oxygen pump” and “nerst cell” that allow the sensor to provide a much more exact reading of the actual air/fuel ratio, not just a rich or lean indication as is the case with a traditional O2 sensor. Instead of generating a voltage signal, an A/F sensor modulates a reference signal from the PCM. The current signal from the A/F sensor can travel in one of two directions (positive or negative), and it gradually increases in the positive direction as the air/fuel mixture becomes leaner. At the “stoichiometric” or “lambda” point when the air/fuel mixture is perfectly balanced (14.7 to 1), the current flow in the A/F sensor circuit stops and there is no current flow
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March 2014 | Counterman
in either direction. When the air/fuel ratio becomes progressively richer, the current reverses direction and gradually increases in the negative direction in proportion to the actual air/fuel ratio. Like ordinary thimble and planar O2 sensors, A/F sensors also have an internal heater circuit to help them reach and maintain the required internal operating temperature. A/F sensors typically operate at a higher temperature: 1,292 to 1,472 degrees F versus about 450 to 600 degrees F for ordinary O2 sensors. Consequently, if the heater circuit fails, the A/F sensor may not produce a reliable signal. This will usually set a fault code and turn on the Check Engine light. Note: The presence of an O2 sensor heater circuit code does not al-
ways mean the sensor needs to be replaced. The fault might be in the external sensor wiring, connector or heater control circuit. O2 sensor problems are one of the leading causes of emission test failures. What’s more, a bad O2 sensor (upstream or downstream) will prevent the OBD II catalyst monitor from running — which is also grounds for rejecting a vehicle for a plug-in OBD II test. The OBD II system needs good signals from both the upstream O2 or A/F sensors and downstream O2 sensors to keep tabs on the operating efficiency of the converter. It’s important to make sure a bad A/F sensor has been correctly diagnosed because the price of these sensors is usually several times more than that of a conventional O2 sensor. CM