These sensors & switches monitor temperatures. If faulty they either send signals to the machines electrical Brain which can activates a warning light, shuts down that part of system, or shut down the entire engine all together. Usually these are located in the cylinder block, oil cooler, or in the cooling system. Switches are set up per specific temperatures. Some turn on at a specific temp and some turn off when the area being monitored by the switch reaches that temperature. At that point the brain will activate or deactivate another device such as a fan.