Check Engine Light Flash In 4th Gear ONLY

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.

Mustang5L5 said:

I’d imagine 4th gear loads it well enough for the misfire to appear. If you lug 4th gear at 20mph does it appear?

have you hooked up abd obd2 scanner and looked for codes and pending codes?

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Once I cleared the codes no more have returned. I did check it just in case. Low speeds in 4th does not trigger it no. Seems right around 55mph in 4th is what makes the light flash