Abstract: An electronic thermostat tester monitors the heat rise of the coolant in the radiator of an internal combustion engine as a function of time and provides "defect" signals if the heat does not reach a preselected level (160.degree. F.) in a predetermined time period (eight minutes). The electronic thermostat tester can also be used with bulldozers, buses, tractors and other heavy-duty equipment. When use with heavy-duty engines, the time intervals will be longer. If the preselected heat range is reached within the time period a light indicates that the thermostat is functioning properly. The tester further includes a plurality of indicating lights for thermostat levels (180.degree. F. and 190.degree. F.) above the preselected heat level (160.degree. F.) for engines having different ratings. By using this thermostat tester one does not have to remove the thermostat from an engine to test the thermostat. In addition, the tester may be used to detect clogged engine blocks, radiators and defective automatic fans.