The invention relates to a test sensor and a heater and thermocouple device for use in a test sensor and other assays, such as a diagnostic test sensor or strip and to a method of manufacturing a test sensor and a device and a method of conducting an assay using the test sensor or device. The invention relates to a test sensor comprising a heater and thermocouple device (100), the heater and thermocouple device (100) comprising: a substrate (10); on the substrate (10), a first layer (12) of a first conductive material of a first conductivity comprising: a first thermocouple element (14); a first connector track (16) connected to the first thermocouple element (14); a resistive heater element (20); and in which at least part of the first thermocouple element (14) is comprised of a portion of the resistive heater element (20); a second layer (22) of a second conductive material of a second conductivity comprising: a second thermocouple element (24) in contact with the first thermocouple element (14) (e.g. together with the first thermocouple element (14) forming a thermocouple junction (50)); a second connector track (26) connected to the second thermocouple element (24); two heater connector tracks (36A, 36B) spaced apart and connected to respective portions of the resistive heater element (20), each on a respective side of the thermocouple elements (14, 24).