The corrosivity of engine antifreeze coolants depends on many factors including inhibitor concentration. Degradation of these antifreeze inhibitors occurs with time in service and therefore leads to increased corrosivity. This paper describes a low cost coolant sensor capable of detecting the corrosivity of engine coolant in-situ. The system is based on electrochemical principles and consists of a sensing probe, electronics package, and an indicating device. Electrochemical theory and supporting data for the system will be presented.