1. 2004 Silverado, C1500 LS, 4.8L, 70k+ miles. The tail pipe has black deposits, I know this is a telling sign, is it O2 sensors? Gas mileage and power are good. 2. What is the OEM O2 sensor replace point? Replacing the up stream sensors is more important than the down stream sensors, yes or no? 3. There have been "lean fuel mixture" codes that were corrected by cleaning the MAF & K&N air filter. .