Method and system for diagnosing thermostat fault of engine cooling system

A method for diagnosing a thermostat of an engine cooling system, wherein the thermostat is disposed at an engine coolant path configured to circulate coolant through an engine and a heater core, the heater core being disposed at a rear of an evaporator that is disposed at a heating and cooling path, the method comprising:

• receiving, from a heater control module by a diagnostic module, an operating current value of a blower motor configured to operate a blower, the blower being configured to supply outside air to the evaporator;
• outputting, by the diagnostic module, a power amount applied to the blower motor for a first certain period of time based on the operating current value of the blower motor;
• receiving, by the diagnostic module, a coolant temperature of the coolant;
• performing, by the diagnostic module, a fault diagnosis of the blower motor;
• outputting a faulty state signal of the thermostat based on the power amount being less than a set operating amount;
• outputting a normal state signal of the thermostat based on the power amount being greater than or equal to the set operating amount;
• terminating the fault diagnosis of the blower motor based on a target revolutions per minute (RPM) of the blower motor being less than or equal to a first RPM;
• and
• outputting the faulty state signal of the thermostat based on a deviation between the target RPM and a current RPM exceeding a second RPM for a second certain period of time,
• wherein terminating the fault diagnosis of the blower motor is performed based on the deviation between the target RPM and the current RPM of the blower motor being less than or equal to the second RPM for the second certain period of time.