PURPOSE: To diagnose whether or not a radiator cooling action by means of a radiator fan actually functions.

CONSTITUTION: In a system which controls a radiator fan into those of stoppage and low-high rotational states, at the time of high speed control (S2), when a state that a cooling water temperature Tw exceeds the specified temperature (S3) is continued as long as more than the specified time (S4), it is so diagnozed that there is something wrong with the radiator fan (S5). In addition, at the time of low speed control over the radiator fan (S6), a fact that a rising speed (S7) in a cooling water temperature (w) is so judged (S8) that it is more than the specified value, it is so diagnosed that there is something wrong with the radiator fan.