The work aims to analyze typical defects of automotive automotive oil cooler, find root cause and make corresponding measures to prevent such problems from occurring repeatedly. Scope of faulty vehicles was defined by performing statistical analysis to distribution of the faulty vehicles. Microstructure of faulty position was observed with SEM, corrosion products around were analyzed with EDX and found to be mainly aluminum oxide, and chloride ions were also found. Allowing for previous corrosion theories and contrastive analysis, the corrosion was found to be mainly caused by penetration effect of chloride ions. According to statistical analysis of antifreeze and content measurement of chlorides ions in the antifreeze samples, the corrosion elements came from the coolant fluid. Based on analysis of mixing process, field inspection of depot emergency operation and simulation experiment, it could be determined that chloride ions in disinfectant were introduced into the mixing water in water storage tank in quantity due to improper operation by depot staff. As a result, the antifreeze was highly corrosive. The Cl? content in mixing water samples to be mixed with the antifreeze was considerably higher than normal level. Too high Cl? content in mixing water leads to penetration of passive film on the aluminum alloy, and causes a typical pitting failure of aluminum alloy finally.