These sensors and switches monitor temperatures. If faulty they either send signals to the machine's electrical brain which can activate a warning light shut down that part of the system or shut down the entire engine. 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.