A sensor element includes: an element base including: a ceramic body made of an oxygen-ion conductive solid electrolyte, and having an inlet at one end portion thereof; at least one internal chamber located inside the ceramic body, and communicating with the gas inlet; and an electrochemical pump cell including an outer electrode, an inner electrode facing the chamber, and a solid electrolyte therebetween, and a porous leading-end protective layer covering a leading end surface and four side surfaces in a predetermined range of the element base on the one end portion, wherein the protective layer has an extension extending into the gas inlet and fixed to an inner wall surface of the ceramic body demarcating the gas inlet, and a gap communicating with the gas inlet is located in the protective layer, with demarcated by a portion of the protective layer continuous with the extension.

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.

Provided is a gas sensor element capable of realizing both highly accurate concentration measurement in environments where the concentration of a specific gas in a measurement target gas is high and highly accurate concentration measurement in environments where the concentration is low. A gas sensor according to one aspect of the present invention adjusts a sensor element drive temperature such that the value of cell resistance of a main pump cell is a predetermined value. Further, in the gas sensor according to one aspect of the present invention, the slope of the cell resistance of the main pump cell is larger than the slope of cell resistance of a measurement pump cell.

Provided is a gas sensor element capable of realizing both highly accurate concentration measurement in environments where the concentration of a specific gas in a measurement target gas is high and highly accurate concentration measurement in environments where the concentration is low. A gas sensor according to one aspect of the present invention adjusts a sensor element drive temperature such that the value of cell resistance of a main pump cell is a predetermined value. Further, in the gas sensor according to one aspect of the present invention, the slope of the cell resistance of the main pump cell is larger than the slope of cell resistance of a measurement pump cell.

Provided is a gas sensor element capable of realizing highly accurate concentration measurement in both environments where the concentration of a specific gas in a measurement target gas is high and where the concentration is low. A gas sensor according to one aspect of the present invention determines whether the concentration of a predetermined gas component in a measurement target gas is higher or lower than a predetermined concentration. If it is determined that the concentration is lower, a specific temperature that a sensor element is to reach as a result of being heated by a heater unit is set to be lower than the specific temperature set if it is determined that the concentration is higher.

Provided is a gas sensor element capable of realizing highly accurate concentration measurement in both environments where the concentration of a specific gas in a measurement target gas is high and where the concentration is low. A gas sensor according to one aspect of the present invention determines whether the concentration of a predetermined gas component in a measurement target gas is higher or lower than a predetermined concentration. If it is determined that the concentration is lower, a specific temperature that a sensor element is to reach as a result of being heated by a heater unit is set to be lower than the specific temperature set if it is determined that the concentration is higher.

A gas sensor element comprising a long plate-like element body and a porous protective layer protecting a surface of the element body, wherein the element body has a gas detection part at an end thereof on one end face side in a longitudinal direction, and the protective layer includes an end face part covering the one end face in laminate, side face parts covering side faces connected to the one end face in laminate, and corner parts where two adjacent ones of the end face part and the side face parts meet. An outer surface of one or more of the end face part and the side face parts has a concave shape that is smoothly continuous with the corner parts and configured such that a layer thickness increases toward the corner parts.

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.