A flex fuel engine is capable of operating efficiently on any combination of gasoline and ethanol. However, an engine combustion strategy must adapt quickly to a change in ethanol concentration after a refueling event in order to achieve optimum engine combustion. Typical control systems rely on an exhaust gas oxygen sensor (lambda) to measure changes in oxygen concentration following combustion. This feedback control approach can take five to ten minutes to detect the fuel change and correct the combustion strategy. This relatively long lag time could result in suboptimal engine performance such as a loss of engine power, engine knocking, poor cold start performance, unburned hydrocarbons, and high pollutant emissions. To counter this shortcoming, an on-board flex fuel sensor (FFS) was developed to enable a feed-forward control strategy. The FFS may be installed inline between the fuel tank and fuel injector and measure the fuel prior to it reaching the injector. The FFS sensor estimates the concentration of ethanol in the fuel in real-time using a correlation based on permittivity, conductivity, and temperature of the fuel flowing through the sensor. The FFS is specifically designed for the Brazilian market and is calibrated to measure the anhydrous ethanol and hydrous ethanol contained in gasohol and alcohol respectively. The sensor can accurately estimate the total ethanol concentration (anhydrous plus hydrous) within +/− five volume percent for any combination of gasohol and alcohol. The sensor can operate with a fuel temperature range of −40°C to 95°C. In addition, the FFS provides diagnostic capability and is able to identify and report water contamination to the engine control module.