A system includes an internal combustion engine having a thermostat provided to selectively allow coolant flow through the engine, and a coolant temperature determination device that provides a coolant temperature value. The system further includes a controller structured to functionally execute operations to diagnose the thermostat. The controller determines whether the engine is providing a high output operation. When the engine is providing a high output operation, the controller determines the thermostat is passed in response to the coolant temperature value exceeding a first threshold temperature value.