A gas sensor includes an element body having an oxygen ion conductive solid electrolyte layer and internally provided with a measurement-object gas flow section that introduces a measurement-object gas and allows the gas to flow; a measurement-object gas-side electrode disposed in a portion of the element body, the portion being exposed to the measurement-object gas; and a reference electrode disposed inside of the element body. Let A [μA] be a limiting current when oxygen is pumped from the surroundings of the measurement-object gas-side electrode to the surroundings of the reference electrode with the measurement-object gas introduction section, and B [μA] be a limiting current when oxygen is pumped from the surroundings of the reference electrode to the surroundings of the measurement-object gas-side electrode with the reference gas introduction section, then a ratio A/B is greater than or equal to 0.005.

An O.sub.2 sensor has a sensor element, which includes a solid electrolyte layer and a pair of electrodes, while the solid electrolyte layer is interposed between the electrodes. The O.sub.2 sensor outputs an electromotive force signal in response to an air-to-fuel ratio of exhaust gas of an engine, which serves as a sensing subject. A constant current circuit, which induces a flow of a predetermined constant electric current between the pair of electrodes of a sensor element, and a current sensing arrangement, which senses a current value of an actual electric current that is conducted through the sensor element, are provided. A microcomputer determines whether an abnormality of the constant current circuit is present based on the current value of the electric current, which is sensed with the current sensing arrangement, in a case where the constant current is induced by the constant current circuit.

An O.sub.2 sensor has a sensor element, which includes a solid electrolyte layer and a pair of electrodes, while the solid electrolyte layer is interposed between the electrodes. The O.sub.2 sensor outputs an electromotive force signal in response to an air-to-fuel ratio of exhaust gas of an engine, which serves as a sensing subject. A constant current circuit, which induces a flow of a predetermined constant electric current between the pair of electrodes of a sensor element, and a current sensing arrangement, which senses a current value of an actual electric current that is conducted through the sensor element, are provided. A microcomputer determines whether an abnormality of the constant current circuit is present based on the current value of the electric current, which is sensed with the current sensing arrangement, in a case where the constant current is induced by the constant current circuit.

A broadband lambda probe includes a measurement hollow space. The broadband lambda probe further includes an oxygen pump cell having an outer pump electrode and an inner pump electrode to enable the transfer of oxygen from the measurement hollow space to an external environment of the broadband lambda probe. The broadband lambda probe further includes a Nernst concentration cell. The broadband lambda probe further includes at least one capacitive sensor device. The at least one capacitive sensor device has a capacitance configured to be varied with a change of a concentration of at least one substance. The concentration is present in the respective at least one capacitive sensor device. The at least one capacitive sensor device is positioned in the broadband lambda probe to at least one of directly adjoin the measurement hollow space and partially project into the measurement hollow space.

A broadband lambda probe includes a measurement hollow space. The broadband lambda probe further includes an oxygen pump cell having an outer pump electrode and an inner pump electrode to enable the transfer of oxygen from the measurement hollow space to an external environment of the broadband lambda probe. The broadband lambda probe further includes a Nernst concentration cell. The broadband lambda probe further includes at least one capacitive sensor device. The at least one capacitive sensor device has a capacitance configured to be varied with a change of a concentration of at least one substance. The concentration is present in the respective at least one capacitive sensor device. The at least one capacitive sensor device is positioned in the broadband lambda probe to at least one of directly adjoin the measurement hollow space and partially project into the measurement hollow space.

A gas sensor includes a sensor device, a device-side insulator porcelain, an atmospheric side insulator porcelain, a housing, a seal disposed between the housing and the device-side insulator porcelain, and an atmospheric side cover. The atmospheric side cover includes a large-diameter portion, a small-diameter portion, and a shoulder portion formed therebetween. The shoulder portion presses a base end surface of the atmospheric side insulator porcelain to a front end side through a biasing member to place the atmospheric side insulator porcelain in contact with the device-side insulator porcelain. The shoulder portion is defined by a contact portion placed in contact with the biasing member and a detached portion separate from the biasing member to form a communication path which communicates between an outer space and an outside path. This avoids entry of measurement gas into an air atmosphere within the atmospheric side insulator porcelain to obtain correct sensor outputs, also avoids breakage of the atmospheric side insulator porcelain, and reduces the manufacturing costs.

A gas sensor includes a sensor device, a device-side insulator porcelain, an atmospheric side insulator porcelain, a housing, a seal disposed between the housing and the device-side insulator porcelain, and an atmospheric side cover. The atmospheric side cover includes a large-diameter portion, a small-diameter portion, and a shoulder portion formed therebetween. The shoulder portion presses a base end surface of the atmospheric side insulator porcelain to a front end side through a biasing member to place the atmospheric side insulator porcelain in contact with the device-side insulator porcelain. The shoulder portion is defined by a contact portion placed in contact with the biasing member and a detached portion separate from the biasing member to form a communication path which communicates between an outer space and an outside path. This avoids entry of measurement gas into an air atmosphere within the atmospheric side insulator porcelain to obtain correct sensor outputs, also avoids breakage of the atmospheric side insulator porcelain, and reduces the manufacturing costs.

A gas sensor includes a sensor element and a control unit for controlling the sensor element. The sensor element includes a main pump cell, an auxiliary pump cell, a measurement pump cell, and a reference electrode, wherein, in the main pump cell, a repeatedly on-off controlled main pump current is applied so that an auxiliary pump current flowing through the auxiliary pump cell is at a predetermined target current value, and, in the auxiliary pump cell, the auxiliary pump current is applied so that an electromotive force between an inner auxiliary pump electrode and the reference electrode is at a predetermined target voltage value. The control unit includes: a control power supply for applying the repeatedly on-off controlled main pump current; and a setting part for setting the target voltage value based on an electric potential difference generated between the inner main pump electrode and the reference electrode.

A gas sensor includes a sensor element and a control unit for controlling the sensor element. The sensor element includes a main pump cell, an auxiliary pump cell, a measurement pump cell, and a reference electrode, wherein, in the main pump cell, a repeatedly on-off controlled main pump current is applied so that an auxiliary pump current flowing through the auxiliary pump cell is at a predetermined target current value, and, in the auxiliary pump cell, the auxiliary pump current is applied so that an electromotive force between an inner auxiliary pump electrode and the reference electrode is at a predetermined target voltage value. The control unit includes: a control power supply for applying the repeatedly on-off controlled main pump current; and a setting part for setting the target voltage value based on an electric potential difference generated between the inner main pump electrode and the reference electrode.

A gas sensor includes an element body, a pump cell, an impedance measurer, and a calculation unit. The pump cell has an inner electrode disposed in a measurement-object gas flow section of the element body, and an outer electrode disposed outside an element body to come into contact with a measurement-object gas, the pump cell being configured to adjust an oxygen concentration in a vicinity of the inner electrode. The impedance measurer performs first measurement to measure a first impedance by applying a voltage having a first frequency to the pump cell, and second measurement to measure a second impedance by applying a voltage having a second frequency higher than the first frequency to the pump cell. The calculation unit calculates the reaction resistance index correlated with the reaction resistance of the pump cell, based on the first and second impedances.