2009 Sierra 4.8 engine, bone stock, 290,000 Km. Started running rough and set P0300 code. Scanner show misfire counter for cylinder 3 & 4 only. I had coils, wires and plugs on hand so changed them on both cylinders, no change. No vacuum leaks. The scan is showing voltages for the O2 sensors in the correct range but both STFT and LTFT show zero. Are O2 sensors bad or is it a wiring issue. If the O2 sensors are bad why only affect 2 cylinders? I?m stumped.