Abstract: A sensor having a temperature dependent resistor mounted within a protective housing. The resistor is connected to external circuitry via a pair of connecting wires. In use, variations in temperature of the exhaust gas cause a variation in the temperature experienced by the resistor and consequently, the resistance of the resistor also varies. The resistor is mounted in a projecting end portion of the housing which is of narrower cross-section than the bulk of the housing. The housing is filled with a filler material. The filler material provides support to the resistor and the wires. In the present invention, unlike in prior art sensors, the filler material does not completely fill the housing. Instead a gap is provided which is crossed by the wires. The gap interrupts heat flow through the filler material and thus improves the thermal isolation of the resistor and thereby the accuracy of the sensor. In an alternative embodiment, the sensor additionally comprises an additional heat exchange element.