Oxygen Sensor Anatomy

Did you ever wonder where a chemistry or physics class could come in handy in the “real world”? The knowledge from these studies can help you understand a problem with a fuel delivery system.

For instance, the oxygen sensor was originally called a Lambda sensor. The Greek letter Lambda is used to describe the voltage range of the sensor when it compares the quantity of oxygen in the exhaust relative to oxygen in the atmosphere. The sensor is made of Zirconium Oxide (ZrO2), a chemical compound used to form the sensor’s thermal-driven electrochemical fuel cell. Two Platinum (Pt) electrodes are placed on the ZrO2 to provide a connection for output voltage to a control module. A reading of 800 mV DC represents a rich mixture where there is little or no oxygen in the exhaust stream. An output voltage of 200 mV DC represents a lean mixture where there is a lot of oxygen in the exhaust stream. The ideal reading is 450 mV DC; this is where the quantities of air and fuel are in the optimum ratio, which is called stoichiometric.

The controller uses 450 mV as a midpoint in a voltage range to control fuel trim for the injector pulse cycle. The sensor’s analog input to the controller is converted to a digital rich or lean command to drive a fuel trim software program. Sometimes referred to as “Block Learn,” it adjusts the cycle time of the fuel injector. The voltage generated by the sensor must be greater or less than the voltage of the damping zone to send a rich or lean signal to the controller. The damping zone acts like a shock absorber on a suspension to prevent the voltage signal from oscillating.

A Planar Air Fuel Sensor is a combination of a standard Zirconium Oxide Oxygen sensor and a Pump Cell to maintain a constant sensing of a stoichiometric air fuel ratio through the extreme rich and lean conditions. The pump cell is a diffusion gap in the Zirconium Oxide of the sensor that is connected to a control circuit.

The pump cell controls the oxygen concentration of the sensor by adding or subtracting oxygen to the diffusion gap. Input to the electronic circuit modifies the oxygen concentration by changing the polarity of the current flow in the pump cell. The changing polarity of the input and trim current flow causes the control circuit to send a rich or lean signal to the engine control module.