I have a 1999 Silverado and the service light keeps coming on. The truck has 137,000 miles on it I had changed the spark plugs when all this started. I have change the mass air flow sensor, new fuel filter all new oxygen sensors and have checked for any vac leaks using soap and water and sprayed carb cleaner around the intake. The truck runs great and idles good and the gas mileage was poor before I had changed the spark plugs and at that time the gas mileage went up and it still is running good. Its just the service engine light will no go off. Codes received are P0171 and P0174. Both banks too lean… any ideas?