Generally, a gas sensor used for an exhaust gas purification system of an internal combustion engine includes a chamber into which exhaust gas containing NOx is introduced, a pump cell disposed on the upstream side within the chamber for pumping out oxygen in the exhaust gas from the chamber, and a sensor cell disposed on the downstream side within the chamber for detecting a NOx concentration in the exhaust gas whose oxygen concentration has been reduced. The pump cell and the sensor cell of the gas sensor are formed such that a pair of electrodes are formed in a solid electrolyte sheet having oxygen ion conductivity and constituting a chamber wall, and a sensor element is formed by stacking a heater sheet and an insulating sheet on the solid electrolyte sheet. Other than the gas sensor of such 2-cell structure, there is known a gas sensor of a three-cell structure in which a sensor cell and a monitor cell are disposed side by side on the downstream side of a chamber to monitor the oxygen concentration in the chamber (for example, refer to patent literature 1 and so on).
Patent literature 1 describes a principle of detection of NOx by a gas sensor, in which exhaust gas is introduced into a chamber as an inner space of a stacked-type sensor element, and oxygen that obstructs detection is discharged to the outside from the chamber while the exhaust gas passes through the pump cell on the upstream side. At this time, when a voltage applied to the pump cell is set such that the current value of a current flowing by the oxygen within the chamber does not depend on the voltage value, it operates as a limiting current-type oxygen sensor. Thereafter, NOx in the exhaust gas whose oxygen concentration has been reduced is decomposed in the sensor cell on the downstream side, so that the NOx concentration can be detected from the value of a current that occurs at that time. Further, by providing the monitor cell to detect a remaining oxygen concentration in the exhaust gas that has reached the sensor cell, the voltage applied to the pump cell can be feed-back controlled.
Incidentally, the gas sensor of the two-cell or three-cell structure is known as an oxygen sensor (for example, see patent literature 2 and so on).