Why is my focus st running lean?
Why is my focus st running lean? This would be a too much air scenario. You could also have a fuel filter clog or problem with one of the fuel pumps. That would be a too little fuel scenario. Could also be a malfunction with the O2 sensor. Should a car run lean or rich? The expression running rich or lean is in reference to the amount of gas that you have in the engine. If there is too much gas, then you will be running rich, and if there isn't enough gas, then you are running lean. Either situation...