a method and apparatus for detecting the presence and quantity of fuel in a fuel / air mixture to the cylinders of a fuel injection engine is disclosed . a portion of the fuel / air mixture injected to the engine is diverted through a solid state sensor which , when included in a detection circuit , provides a resistance which is proportional to the amount of fuel in the mixture . the sensor reacts to the presence of the fuel vapors by providing a change in the voltage across the sensor .