97 expedition 4.6L 138,000 trouble free miles. P0402 & P0304 popped up the other day. P0402 is a repeat offender of 2 months ago. I new dpfe and upper air intake cleaning cured the first apperance. #4 cylinder plug appears dry, however I don't think the wires were ever changed, so new ones are on the way.
Any other suggestions on where to start? Thanks