PURPOSE: A cold starting estimating method and a cold starting detecting sensor of a fuel cell system are provided to decide whether cooling water is frozen or not while consuming relatively small energy and to obtain semi-permanent durability. CONSTITUTION: A cold starting estimating method of a fuel cell system comprises the following steps: a voltage application step(S101) sanctioning AC voltage between two electrode installed in a sample solution which is pure water having same phase as cooling water of the fuel cell system; a current measurement step(S102) measuring the AC voltage flowing in the sample solution using the two electrodes; a specified value calculation step(S103) calculating a specified value of resistance, inductance, or capacitance of the sample solution; and decision step(S104) deciding whether the cooling water is frozen or not by comparing the calculated specified value with a specified value corresponding to the frozen sample solution.
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