Systems and methods are disclosed for determining a temperature of a coolant in a cooling system for an engine and diagnosing a thermostat in the engine responsive to the determined temperature. The system and method includes interpreting engine heat data indicative of a first amount of heat introduced into the internal combustion engine; interpreting exhaust gas recirculation (EGR) heat data indicative of a second amount of heat introduced into the internal combustion engine via the amount of exhaust gas provided to the intake manifold; interpreting heat loss data indicative of an amount of heat loss experienced by the coolant; determining a temperature of the coolant based on the first amount of heat, the second amount of heat, and the amount of heat loss; comparing the determined temperature of the coolant to a sensed temperature of the coolant; and determining a status of the thermostat responsive to the comparison.