A method includes providing a system having a fluid flow, a fuel injector and an oxygen sensor disposed in the fluid flow, where the oxygen sensor is downstream of the fuel injector. The method includes determining a first air fuel ratio, changing an injection rate of the fuel injector and determining a second air fuel ratio, and determining a fault value for the fuel injector from the first air fuel ratio and the second air fuel ratio. The method further includes determining the fault value for the fuel injector by determining a difference between the first air fuel ratio and the second air fuel ratio, and by determining that the fault value is positive in response to the difference being lower than a passing threshold value. The method includes changing injection rates of the fuel injector for specified periods of time short enough to significant disruption of system temperatures.