how many miles on the truck? o2 sensors are considered a tune up item and if they are old they will get lazy, you could get a good voltage reading one day and nothing the next. I still would do a good maf cleaning, remove it and spray it down with the cleaner, don't wipe it or stick anything in there just spray the heck out of it. the intake bolts are a known problem and a cause for leaks, the fix has cured many 1131,1151 code problems. best of luck.