A gas sensor includes a sensor element and a controller. The sensor element includes an element body provided with a measurement-object gas flow section therein, a measurement electrode disposed in the measurement-object gas flow section, an outer pump electrode provided in the element body so that the outer pump electrode comes into contact with a measurement-object gas, a reference electrode, a reference-gas introduction section that causes a reference gas to flow to the reference electrode, and a reference-gas adjustment pump cell constituted by including the outer pump electrode and the reference electrode. The controller performs a moisture-absorption-state diagnosis process of diagnosing a moisture absorption state around the reference electrode based on a pump current flowing through the reference-gas adjustment pump cell when the reference-gas adjustment pump cell is controlled to pump out oxygen from a periphery of the reference electrode to a periphery of the outer pump electrode.

A sensor element according to the present invention includes an element body having a measurement-object gas flow section into which an exhaust gas is introduced, an adjustment pump cell including a measurement-object-gas-side electrode disposed in a portion exposed to the exhaust gas on an outer side of the element body, the adjustment pump cell being configured to adjust an oxygen concentration in an oxygen concentration adjustment chamber included in the measurement-object gas flow section, a measurement electrode disposed in a measurement chamber located downstream of the oxygen concentration adjustment chamber, and a reference electrode into which a reference gas is introduced. The measurement-object-gas-side electrode has an Au/(Pt+Au) ratio (=an area of a portion where Au is exposed/an area of a portion where Au and Pt are exposed) 0.2-0.7, the Au/(Pt+Au) ratio being measured by using XPS.

A gas sensor element comprises a solid electrolyte layer; a detection electrode provided on one surface of the solid electrolyte layer; a reference electrode provided on another surface of the solid electrolyte layer; a first layer provided on a side where the other surface of the solid electrolyte layer is present, and having a reference gas flow path; and a heater layer provided on a side opposite to a side where the solid electrolyte layer is provided. In the gas sensor element, an introduction flow path is formed as a flow path for guiding the reference gas from outer surfaces of the gas sensor element to the reference gas flow path. The introduction flow path has an opening provided at an outer surface that is perpendicular to a stacking direction of the gas sensor element and is provided on a side opposite to the heater layer.

Disclosed is a gas sensor element having an electrode containing a first metal as a predominant component and a lead containing a second metal as a predominant component. The electrode and the lead are connected directly at a connection boundary thereof, or connected indirectly via a connection joint. The connection boundary or joint includes a component region where either one of the first and second metals lower in specific gravity than the other of the first and second metals is contained in an amount ranging between those in the electrode and the lead.

An NO.sub.2 sensor comprises a sensing electrode material that includes a compound consisting essentially of one or more elements selected from the group consisting of Mn, Co, and Fe; an element selected from the group consisting of Si and Ti; and oxygen. In an alternate embodiment, the sensing electrode material includes a compound consisting essentially of one or more elements selected from the group consisting of Mn, Co, and Fe; an element selected from the group consisting of Si and Ti; an element selected from the group consisting of Mg, Al, Li, Na, and K; and oxygen. Sensors made with these sensing electrode materials demonstrate good NO.sub.2 sensitivity and reduced sensitivity to cross-interference from NO and NH.sub.3.

An ammonia detection section is disposed on an electrically insulating layer and includes a reference electrode, a solid electrolyte body for ammonia, and a detection electrode that are stacked in this order on the electrically insulating layer. In the ammonia detection section, a three-phase boundary is formed at the interface between the reference electrode and the solid electrolyte body for ammonia, and another three-phase boundary is formed at the interface between the detection electrode and the solid electrolyte body for ammonia. The concentration of ammonia in exhaust gas is thereby detected. The ammonia detection section includes a porous layer formed of an electrically insulating porous material and disposed between the insulating layer and the reference electrode.

A CO sensor includes a solid electrolyte substrate, a sensing electrode, and a reference electrode, and outputs electromotive forces in accordance with CO concentrations. The sensing electrode and the reference electrode are provided on the same surface of the solid electrolyte substrate. The sensing electrode contains a metal oxide such as Bi.sub.2O.sub.3 that generates a positive electromotive force response when coming into contact with CO. The reference electrode contains a metal oxide such as CeO.sub.2 that generates a negative electromotive force response when coming into contact with CO.

A hydrogen detector for a gaseous medium is disclosed. The detector includes an operating element fixed to the upper part of the detector housing by means of sealant. The lower part of the detector is insulated and in contact with a heater that provides operational temperature of the medium supplied to a waterproof membrane of a steam hydrogen compartment. Disturbances introduced by a measurement flow is transferred to the central core of a potential measuring unit through a measuring platinum electrode fixed to the lower part of a ceramic sensing element connected to the metal casing of the sensing element by the sealant. A standard electrode is located in the inner cavity of the ceramic sensing element. The external part of the ceramic sensing element bottom is covered with a porous platinum electrode. The end of the potential measuring unit central core is brought out to the standard electrode.

An electrochemical sensor is provided having a housing with an opening therein. The housing defines a chamber. The sensor includes a primary cell having a primary working electrode aligned with the opening in the housing so that the primary working electrode is exposed to an environment outside of the chamber. A primary counter electrode is sealed within the chamber. The sensor includes a secondary cell having a secondary working electrode sealed within the chamber, and a secondary counter electrode sealed within the chamber.

The present invention provides a gas sensor capable of suppressing a variation of a sensor output by a sensor cell to be small. A sensor element of a gas sensor comprises a first solid electrolyte body and a second solid electrolyte body having oxygen ion conductivity, a measured gas chamber into which the measured gas is introduced, a first reference gas chamber and a second reference gas chamber into which reference gas is introduced, a first pump cell, a second pump cell, a sensor cell, and a heater. A value obtained by dividing a first average cross-sectional area of the first reference gas chamber by the first length is larger than the value obtained by dividing the second average cross-sectional area of the second reference gas chamber by the second length.