no. 5 coil problem

[DALE]

going to work my 02 pathfinder se 4x4 3.5 started sputtering and skipping 1at highway speed. every time i tried to accelerate it became much worse. took it to Advance. his computer said the number 5 plug is not firing properly and "manufacturer air fuel metering". The truck has 140,000 miles on it and I dont know if the previous owner ever changed the plugs. I bought the platinum plugs and did the passenger side with no problem. I was hoping once I replaced the no. 5 plug that would cure the problem but it didnt. Now I have to do the hard side. If I go through the headache of doing the hard side and the problem persists would that mean that it is the no. 5 COIL causing my truck to fall on its face and sputter on acceleration??? when it sputters on acceleration the service engine light flashes then just stays on after the engine smooths out w/o any accleration. I would hate to spend 100.00 dollars on a coil pack and that NOT be the problem. ANY HELP WOULD BE GREATLY APPRECIATED!!!