Operation of the probe

The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a lean mixture. That is one where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). A reading of 0.8 V (800 mV) DC represents a rich mixture, one which is high in unburned fuel and low in remaining oxygen. The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output mainly consists of fully oxidized CO2.

The voltage produced by the sensor is so nonlinear with respect to oxygen concentration that it is impractical for the engine control unit (ECU) to measure intermediate values - it merely registers "lean" or "rich", and periodically adjusts the fuel/air mixture to keep the output of the sensor alternating between these two states. The time period chosen by the ECU to monitor the sensor and adjust the fuel/air mixture creates an inevitable delay, which makes this system less responsive than one using a linear sensor (see below). The shorter the time period, the higher the so-called "cross count"and the more responsive the system.

The zirconia sensor is of the 'narrow band' type, referring to the narrow range of fuel/air ratios to which it responds.