the lambda sensor , shown as a detail and partly in section in fig1 as an exemplary embodiment for a component in general that is exposed to a thermal overload serves to define the oxygen content in the exhaust gas of the internal combustion engine of a motor vehicle . it has a metal housing 11 , which has a thread 12 as a fastening means for incorporation into an exhaust pipe of the engine and on which a hexagonal nut 13 is embodied for installing the lambda sensor . a longitudinal bore is provided in the housing 11 , and a platelike sensor element 14 passes through this bore . on the side toward the gas to be measured , the sensor element 14 has an end portion 141 that is exposed to the exhaust gas and on the connection side an end portion 142 where the sensor element 14 is made to contact electrical connection lines . the sensor element 14 is received in gastight fashion in the longitudinal bore by means of two ceramic molded parts and a seal between them . of the ceramic molded parts , only the ceramic molded part 15 can be seen in fig4 ; this is the ceramic molded part oriented toward the end portion 142 , on the connection side , of the sensor element 14 . the end portion 141 , on the side toward the gas to be measured , of the sensor element 14 protrudes out from the housing 11 and there is surrounded by a guard tube 16 , which is affixed to the housing 11 . the guard tube 16 has inlet and outlet openings 17 for the exhaust gas to be measured . integrated with the lambda sensor is an indicator 20 , which permanently indicates a thermal overload — even if only temporary — on the lambda sensor from excessively hot exhaust gas . to that end , a blind bore 18 which is filled with an indication material 21 is made in the hexagonal nut 13 ( fig2 ). this indication material 21 has a limit temperature which changes its microscopic structure when exceeded , for instance as the result of a melting process . the limit temperature of the indication material is adapted to the maximum allowable operating temperature of the lambda sensor , or in other words of the sensor element 14 , in such a way that it either corresponds to that operating temperature or is only slightly above it . if the lambda sensor is then thermally overloaded during its operation , the result of which ranges from an impairment in the function of the sensor element 14 to the complete failure of the lambda sensor , then after the limit temperature is exceeded , the microstructure of the indication material 21 also changes . this microstructural change can be ascertained quickly and simply , for instance from the visual appearance . thus a fast diagnosis as to whether a malfunction or failure of the lambda sensor can be ascribed to a thermal overload , or not , can be made . in the exemplary embodiment shown , the indication material is a ceramic foam , which is forced into the blind bore 18 . the ceramic foam is made up of approximately 68 vol -% ( volume percent ) of an aluminum - magnesium alloy ( almg 5 ) and about 32 vol -% of magnesium oxide ( mgo ). such a ceramic foam , in a temperature range that is below the limit temperature of approximately 650 ° c ., has a microstructure that is schematically sketched in fig3 . if this limit temperature is exceeded , a visually apparent sweating out of aluminum beads 22 occurs , and these beads can readily be seen on the surface ( fig4 ). this sweating out of aluminum beads 22 occurs up to a second limit temperature of approximately 750 ° c . if this second limit temperature is exceeded , the outflowing aluminum enters into a chemical reaction with the magnesium oxide , leading to a so - called reaction - sintered magnesium spinel ( mgal 2 o 4 ), whose structure is shown schematically in fig5 . since in both cases ( fig4 and fig5 ) the microstructural change of the ceramic foam is permanent , it can be proven quickly , reliably and unambiguously and at little effort or expense for measurement whether the lambda sensor was exposed — even if only temporarily — to a thermal overload of approximately 650 ° c . to approximately 750 ° c . ( fig4 ), or to a thermal load of over approximately 750 ° c . as the indication material , aluminum can also be used , for instance in powder form , and forced into the blind bore 18 . the pressing is sufficient to fix the indicator 20 firmly in the blind bore 18 . if there is a loose powder filling , however , the blind bore 18 can additionally be closed with an easily removed layer . with this kind of powder compaction of aluminum , a limit temperature of approximately 650 ° c . can once again be established . a precise adaptation in terms of the limit temperature at which a thermal overload is to be detected is made possible by using aluminum alloys , such as alloys of aluminum and silicon or aluminum and magnesium . for indicating or detecting thermal overload in a temperature range & lt ; 500 ° c ., plastics which form thermoplastics when the limit temperature is exceeded are used as the indication material . one such plastic is polypropylene , or ptfe , with a detection temperature range from 300 ° c . to 350 ° c . the foregoing relates to preferred exemplary embodiments of the invention , it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention , the latter being defined by the appended claims .