with the working temperature of a catalytic converter. The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use the metal case as ground for the sensor element signal, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires. Operation of the probe Zirconia sensor 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" of fuel and oxygen, 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). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", which is high in unburned fuel and low in remaining oxygen. The ideal setpoint is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimal ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide. The voltage produced by the sensor is nonlinear with respect to oxygen concentration. The sensor is most sensitive near the stoichiometric point (where λ = 1) and less sensitive when either very lean or very rich. The ECU is a control system that uses feedback from the sensor to adjust the fuel/air mixture. As in all control systems, the time constant of the sensor is important; the ability of the ECU to control the fuel–air ratio depends upon the response time of the sensor. An aging or fouled sensor tends to have a slower response time, which can degrade system performance. The shorter the time period, the higher the so-called "cross count" and the more responsive the system. The sensor has a rugged stainless-steel construction internally and externally. Due to this the sensor has a high resistance to corrosion, allowing it to be used effectively in aggressive environments with high temperature/pressure. The zirconia sensor is of the "narrow-band" type, referring to the narrow range of fuel/air ratios to which it responds. Wideband zirconia sensor A variation on the zirconia sensor, called the "wideband" sensor, was introduced by NTK in