I have a 1999 7.4l vortec c3500. I have a code that keeps popping for mass airflow sensor. Replaced MAS, changed pigtail, air filter, oil change, changed oem fuel filter since it was due to be changed, changed fuel pump since it failed, and o2 sensors since it was needed to be replaced due to new flowmaster exhaust. Can’t figure it out. Any theories ladies and gentlemen?