Had a CEL on for misfire cyl 4. Replaced all 8 plugs and cyl 4 coil. Reset engine light. Been driving it a bit since then. Now in 4th gear ONLY the CEL flashes, shift to 5th and it goes away. I don’t know what the hell is going on. Weird. 2002 Mustang GT. 4.6 with SLP Long Tube Headers, Cats are welded in, SLP loud mouth cat back exhaust. Cold air intake. 3.73 gear swap.