A gas sensor includes a detection circuit unit that detects a specific gas component in measured gas based on output from a sensor element. The detection circuit unit includes an AC voltage application unit that applies an AC voltage signal to a pair of electrode units in an electrochemical cell, a gas concentration detection unit that detects concentration information on the specific gas component from a DC signal component included in an output signal provided by the electrochemical cell, and a cell temperature detection unit that detects temperature information on the electrochemical cell from an AC signal component included in the output signal. The cell temperature detection unit includes a signal extraction unit that removes the DC signal component to separate the AC signal component from the output signal, and a synchronous detection unit that performs synchronous detection on the separated AC signal component using the AC voltage signal.

A gas sensor includes a detection circuit unit that detects a specific gas component in measured gas based on output from a sensor element. The detection circuit unit includes an AC voltage application unit that applies an AC voltage signal to a pair of electrode units in an electrochemical cell, a gas concentration detection unit that detects concentration information on the specific gas component from a DC signal component included in an output signal provided by the electrochemical cell, and a cell temperature detection unit that detects temperature information on the electrochemical cell from an AC signal component included in the output signal. The cell temperature detection unit includes a signal extraction unit that removes the DC signal component to separate the AC signal component from the output signal, and a synchronous detection unit that performs synchronous detection on the separated AC signal component using the AC voltage signal.

A first gas sensor having a first sensor element includes a first protective cover that protects the first sensor element, and a second gas sensor having a second sensor element includes a second protective cover that protects the second sensor element. The first protective cover is coated with an oxidation catalyst for one target component from among a plurality of target components, and the second protective cover is coated with an inert catalyst for the one target component.

A first gas sensor having a first sensor element includes a first protective cover that protects the first sensor element, and a second gas sensor having a second sensor element includes a second protective cover that protects the second sensor element. The first protective cover is coated with an oxidation catalyst for one target component from among a plurality of target components, and the second protective cover is coated with an inert catalyst for the one target component.

A NOx sensor element includes: a first pump cell configured to adjust an oxygen concentration in a first measurement chamber; a diffusion resistance portion configured to adjust a diffusion rate of a measurement target gas introduced into the first measurement chamber; and a second pump cell in which a pump current corresponding to a NOx concentration in the measurement target gas after the adjustment of the oxygen concentration, flows. The first pump cell includes: a first solid electrolyte; an inner pump electrode containing a noble metal, and exposed to the first measurement chamber; and an outer pump electrode containing a noble metal, and disposed outside the first measurement chamber. The outer pump electrode contains not less than 22% by mass of a main component of the first solid electrolyte.

A NOx sensor element includes: a first pump cell configured to adjust an oxygen concentration in a first measurement chamber; a diffusion resistance portion configured to adjust a diffusion rate of a measurement target gas introduced into the first measurement chamber; and a second pump cell in which a pump current corresponding to a NOx concentration in the measurement target gas after the adjustment of the oxygen concentration, flows. The first pump cell includes: a first solid electrolyte; an inner pump electrode containing a noble metal, and exposed to the first measurement chamber; and an outer pump electrode containing a noble metal, and disposed outside the first measurement chamber. The outer pump electrode contains not less than 22% by mass of a main component of the first solid electrolyte.

A sensor element includes element main body including side surfaces, a detection unit, connector electrodes disposed on the rear end-side part of the side surfaces, a porous layer that covers at least front end-side part of the side surface, the porous layer having a porosity of 10% or more, and a water-penetration reduction portion. The water-penetration reduction portion is disposed on the side surface so as to divide the porous layer or to be located closer to the rear end than the porous layer. The length L of the water-penetration reduction portion is 0.5 mm or more. The water-penetration reduction portion includes, among a dense layer covering the side surface and having a porosity of less than 10% and a gap region in which the porous layer is absent, at least the dense layer. The water-penetration reduction portion reduces the capillarity of water.

A sensor element includes element main body including side surfaces, a detection unit, connector electrodes disposed on the rear end-side part of the side surfaces, a porous layer that covers at least front end-side part of the side surface, the porous layer having a porosity of 10% or more, and a water-penetration reduction portion. The water-penetration reduction portion is disposed on the side surface so as to divide the porous layer or to be located closer to the rear end than the porous layer. The length L of the water-penetration reduction portion is 0.5 mm or more. The water-penetration reduction portion includes, among a dense layer covering the side surface and having a porosity of less than 10% and a gap region in which the porous layer is absent, at least the dense layer. The water-penetration reduction portion reduces the capillarity of water.

A sensor element includes an element body including an oxygen-ion-conductive solid electrolyte layer, the element body having a longitudinal direction, a measurement electrode disposed in the element body, a reference electrode disposed in the element body so as to come into contact with a reference gas, and a heater configured to heat the solid electrolyte layer. A center of gravity of the reference electrode overlaps the measurement electrode as viewed in a thickness direction of the solid electrolyte layer. A length of each of the reference electrode and the measurement electrode in a front-rear direction is less than or equal to 1.1 mm, the front-rear direction being a direction along the longitudinal direction of the element body. An area of the reference electrode as viewed in the thickness direction is greater than or equal to 1.0 mm.sup.2.

A sensor element includes an element body including an oxygen-ion-conductive solid electrolyte layer, the element body having a longitudinal direction, a measurement electrode disposed in the element body, a reference electrode disposed in the element body so as to come into contact with a reference gas, and a heater configured to heat the solid electrolyte layer. A center of gravity of the reference electrode overlaps the measurement electrode as viewed in a thickness direction of the solid electrolyte layer. A length of each of the reference electrode and the measurement electrode in a front-rear direction is less than or equal to 1.1 mm, the front-rear direction being a direction along the longitudinal direction of the element body. An area of the reference electrode as viewed in the thickness direction is greater than or equal to 1.0 mm.sup.2.